Detect and Remove Loop in a Linked List - GeeksforGeeks

18 Apr, 2022
Write a function detectAndRemoveLoop() that checks whether a given Linked List contains loop and if loop is present then removes the loop and returns true. If the list doesn’t contain loop then it returns false. Below diagram shows a linked list with a loop. detectAndRemoveLoop() must change the below list to 1->2->3->4->5->NULL.
We also recommend to read following post as a prerequisite of the solution discussed here. Write a C function to detect loop in a linked listBefore trying to remove the loop, we must detect it. Techniques discussed in the above post can be used to detect loop. To remove loop, all we need to do is to get pointer to the last node of the loop. For example, node with value 5 in the above diagram. Once we have pointer to the last node, we can make the next of this node as NULL and loop is gone. We can easily use Hashing or Visited node techniques (discussed in the above mentioned post) to get the pointer to the last node. Idea is simple: the very first node whose next is already visited (or hashed) is the last node. We can also use Floyd Cycle Detection algorithm to detect and remove the loop. In the Floyd’s algo, the slow and fast pointers meet at a loop node. We can use this loop node to remove cycle. There are following two different ways of removing loop when Floyd’s algorithm is used for Loop detection.
Method 1 (Check one by one) We know that Floyd’s Cycle detection algorithm terminates when fast and slow pointers meet at a common point. We also know that this common point is one of the loop nodes (2 or 3 or 4 or 5 in the above diagram). Store the address of this in a pointer variable say ptr2. After that start from the head of the Linked List and check for nodes one by one if they are reachable from ptr2. Whenever we find a node that is reachable, we know that this node is the starting node of the loop in Linked List and we can get the pointer to the previous of this node.
Output:
Linked List after removing loop 
50 20 15 4 10 
Method 2 (Better Solution)  
This method is also dependent on Floyd’s Cycle detection algorithm.Detect Loop using Floyd’s Cycle detection algorithm and get the pointer to a loop node.Count the number of nodes in loop. Let the count be k.Fix one pointer to the head and another to a kth node from the head.Move both pointers at the same pace, they will meet at loop starting node.Get a pointer to the last node of the loop and make next of it as NULL.
This method is also dependent on Floyd’s Cycle detection algorithm.
Detect Loop using Floyd’s Cycle detection algorithm and get the pointer to a loop node.
Count the number of nodes in loop. Let the count be k.
Fix one pointer to the head and another to a kth node from the head.
Move both pointers at the same pace, they will meet at loop starting node.
Get a pointer to the last node of the loop and make next of it as NULL.
Thanks to WgpShashank for suggesting this method.Below image is a dry run of ‘remove loop’ function in the code : 
Below is the implementation of the above approach:
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#include <bits/stdc++.h>using namespace std; /* Link list node */struct Node {    int data;    struct Node* next;}; /* Function to remove loop. */void removeLoop(struct Node*, struct Node*); /* This function detects and removes loop in the list  If loop was there in the list then it returns 1,  otherwise returns 0 */int detectAndRemoveLoop(struct Node* list){    struct Node *slow_p = list, *fast_p = list;     // Iterate and find if loop exists or not    while (slow_p && fast_p && fast_p->next) {        slow_p = slow_p->next;        fast_p = fast_p->next->next;         /* If slow_p and fast_p meet at some point then there           is a loop */        if (slow_p == fast_p) {            removeLoop(slow_p, list);             /* Return 1 to indicate that loop is found */            return 1;        }    }     /* Return 0 to indicate that there is no loop*/    return 0;} /* Function to remove loop. loop_node --> Pointer to one of the loop nodes head -->  Pointer to the start node of the linked list */void removeLoop(struct Node* loop_node, struct Node* head){    struct Node* ptr1 = loop_node;    struct Node* ptr2 = loop_node;     // Count the number of nodes in loop    unsigned int k = 1, i;    while (ptr1->next != ptr2) {        ptr1 = ptr1->next;        k++;    }     // Fix one pointer to head    ptr1 = head;     // And the other pointer to k nodes after head    ptr2 = head;    for (i = 0; i < k; i++)        ptr2 = ptr2->next;     /*  Move both pointers at the same pace,      they will meet at loop starting node */    while (ptr2 != ptr1) {        ptr1 = ptr1->next;        ptr2 = ptr2->next;    }     // Get pointer to the last node    while (ptr2->next != ptr1)        ptr2 = ptr2->next;     /* Set the next node of the loop ending node      to fix the loop */    ptr2->next = NULL;} /* Function to print linked list */void printList(struct Node* node){    // Print the list after loop removal    while (node != NULL) {        cout << node->data << " ";        node = node->next;    }} struct Node* newNode(int key){    struct Node* temp = new Node();    temp->data = key;    temp->next = NULL;    return temp;} // Driver Codeint main(){    struct Node* head = newNode(50);    head->next = newNode(20);    head->next->next = newNode(15);    head->next->next->next = newNode(4);    head->next->next->next->next = newNode(10);     /* Create a loop for testing */    head->next->next->next->next->next = head->next->next;     detectAndRemoveLoop(head);     cout << "Linked List after removing loop \n";    printList(head);    return 0;} // This code has been contributed by Striver
#include <bits/stdc++.h>using namespace std; /* Link list node */struct Node {    int data;    struct Node* next;}; /* Function to remove loop. */void removeLoop(struct Node*, struct Node*); /* This function detects and removes loop in the list  If loop was there in the list then it returns 1,  otherwise returns 0 */int detectAndRemoveLoop(struct Node* list){    struct Node *slow_p = list, *fast_p = list;     // Iterate and find if loop exists or not    while (slow_p && fast_p && fast_p->next) {        slow_p = slow_p->next;        fast_p = fast_p->next->next;         /* If slow_p and fast_p meet at some point then there           is a loop */        if (slow_p == fast_p) {            removeLoop(slow_p, list);             /* Return 1 to indicate that loop is found */            return 1;        }    }     /* Return 0 to indicate that there is no loop*/    return 0;} /* Function to remove loop. loop_node --> Pointer to one of the loop nodes head -->  Pointer to the start node of the linked list */void removeLoop(struct Node* loop_node, struct Node* head){    struct Node* ptr1 = loop_node;    struct Node* ptr2 = loop_node;     // Count the number of nodes in loop    unsigned int k = 1, i;    while (ptr1->next != ptr2) {        ptr1 = ptr1->next;        k++;    }     // Fix one pointer to head    ptr1 = head;     // And the other pointer to k nodes after head    ptr2 = head;    for (i = 0; i < k; i++)        ptr2 = ptr2->next;     /*  Move both pointers at the same pace,      they will meet at loop starting node */    while (ptr2 != ptr1) {        ptr1 = ptr1->next;        ptr2 = ptr2->next;    }     // Get pointer to the last node    while (ptr2->next != ptr1)        ptr2 = ptr2->next;     /* Set the next node of the loop ending node      to fix the loop */    ptr2->next = NULL;} /* Function to print linked list */void printList(struct Node* node){    // Print the list after loop removal    while (node != NULL) {        cout << node->data << " ";        node = node->next;    }} struct Node* newNode(int key){    struct Node* temp = new Node();    temp->data = key;    temp->next = NULL;    return temp;} // Driver Codeint main(){    struct Node* head = newNode(50);    head->next = newNode(20);    head->next->next = newNode(15);    head->next->next->next = newNode(4);    head->next->next->next->next = newNode(10);     /* Create a loop for testing */    head->next->next->next->next->next = head->next->next;     detectAndRemoveLoop(head);     cout << "Linked List after removing loop \n";    printList(head);    return 0;} // This code has been contributed by Striver
// Java program to detect and remove loop in linked list class LinkedList {     static Node head;     static class Node {         int data;        Node next;         Node(int d)        {            data = d;            next = null;        }    }     // Function that detects loop in the list    int detectAndRemoveLoop(Node node)    {        Node slow = node, fast = node;        while (slow != null && fast != null && fast.next != null) {            slow = slow.next;            fast = fast.next.next;             // If slow and fast meet at same point then loop is present            if (slow == fast) {                removeLoop(slow, node);                return 1;            }        }        return 0;    }     // Function to remove loop    void removeLoop(Node loop, Node head)    {        Node ptr1 = loop;        Node ptr2 = loop;         // Count the number of nodes in loop        int k = 1, i;        while (ptr1.next != ptr2) {            ptr1 = ptr1.next;            k++;        }         // Fix one pointer to head        ptr1 = head;         // And the other pointer to k nodes after head        ptr2 = head;        for (i = 0; i < k; i++) {            ptr2 = ptr2.next;        }         /*  Move both pointers at the same pace,         they will meet at loop starting node */        while (ptr2 != ptr1) {            ptr1 = ptr1.next;            ptr2 = ptr2.next;        }         // Get pointer to the last node        while (ptr2.next != ptr1) {            ptr2 = ptr2.next;        }         /* Set the next node of the loop ending node         to fix the loop */        ptr2.next = null;    }     // Function to print the linked list    void printList(Node node)    {        while (node != null) {            System.out.print(node.data + " ");            node = node.next;        }    }     // Driver program to test above functions    public static void main(String[] args)    {        LinkedList list = new LinkedList();        list.head = new Node(50);        list.head.next = new Node(20);        list.head.next.next = new Node(15);        list.head.next.next.next = new Node(4);        list.head.next.next.next.next = new Node(10);         // Creating a loop for testing        head.next.next.next.next.next = head.next.next;        list.detectAndRemoveLoop(head);        System.out.println("Linked List after removing loop : ");        list.printList(head);    }} // This code has been contributed by Mayank Jaiswal
# Python program to detect and remove loop in linked list # Node classclass Node:     # Constructor to initialize the node object    def __init__(self, data):        self.data = data        self.next = None class LinkedList:     # Function to initialize head    def __init__(self):        self.head = None     def detectAndRemoveLoop(self):        slow_p = fast_p = self.head                 while(slow_p and fast_p and fast_p.next):            slow_p = slow_p.next            fast_p = fast_p.next.next             # If slow_p and fast_p meet at some point then            # there is a loop            if slow_p == fast_p:                self.removeLoop(slow_p)                         # Return 1 to indicate that loop is found                return 1                 # Return 0 to indicate that there is no loop        return 0     # Function to remove loop    # loop_node --> pointer to one of the loop nodes    # head --> Pointer to the start node of the linked list    def removeLoop(self, loop_node):        ptr1 = loop_node        ptr2 = loop_node                 # Count the number of nodes in loop        k = 1        while(ptr1.next != ptr2):            ptr1 = ptr1.next            k += 1         # Fix one pointer to head        ptr1 = self.head                 # And the other pointer to k nodes after head        ptr2 = self.head        for i in range(k):            ptr2 = ptr2.next         # Move both pointers at the same place        # they will meet at loop starting node        while(ptr2 != ptr1):            ptr1 = ptr1.next            ptr2 = ptr2.next         # Get pointer to the last node        while(ptr2.next != ptr1):            ptr2 = ptr2.next         # Set the next node of the loop ending node        # to fix the loop        ptr2.next = None     # Function to insert a new node at the beginning    def push(self, new_data):        new_node = Node(new_data)        new_node.next = self.head        self.head = new_node     # Utility function to print the LinkedList    def printList(self):        temp = self.head        while(temp):            print(temp.data, end = ' ')            temp = temp.next  # Driver programllist = LinkedList()llist.push(10)llist.push(4)llist.push(15)llist.push(20)llist.push(50) # Create a loop for testingllist.head.next.next.next.next.next = llist.head.next.next llist.detectAndRemoveLoop() print("Linked List after removing loop")llist.printList() # This code is contributed by Nikhil Kumar Singh(nickzuck_007)
// A C# program to detect and remove loop in linked listusing System;public class LinkedList {     Node head;     public class Node {         public int data;        public Node next;         public Node(int d)        {            data = d;            next = null;        }    }     // Function that detects loop in the list    int detectAndRemoveLoop(Node node)    {        Node slow = node, fast = node;        while (slow != null && fast != null && fast.next != null) {            slow = slow.next;            fast = fast.next.next;             // If slow and fast meet at same            // point then loop is present            if (slow == fast) {                removeLoop(slow, node);                return 1;            }        }        return 0;    }     // Function to remove loop    void removeLoop(Node loop, Node head)    {        Node ptr1 = loop;        Node ptr2 = loop;         // Count the number of nodes in loop        int k = 1, i;        while (ptr1.next != ptr2) {            ptr1 = ptr1.next;            k++;        }         // Fix one pointer to head        ptr1 = head;         // And the other pointer to k nodes after head        ptr2 = head;        for (i = 0; i < k; i++) {            ptr2 = ptr2.next;        }         /* Move both pointers at the same pace,        they will meet at loop starting node */        while (ptr2 != ptr1) {            ptr1 = ptr1.next;            ptr2 = ptr2.next;        }         // Get pointer to the last node        while (ptr2.next != ptr1) {            ptr2 = ptr2.next;        }         /* Set the next node of the loop ending node        to fix the loop */        ptr2.next = null;    }     // Function to print the linked list    void printList(Node node)    {        while (node != null) {            Console.Write(node.data + " ");            node = node.next;        }    }     // Driver program to test above functions    public static void Main(String[] args)    {        LinkedList list = new LinkedList();        list.head = new Node(50);        list.head.next = new Node(20);        list.head.next.next = new Node(15);        list.head.next.next.next = new Node(4);        list.head.next.next.next.next = new Node(10);         // Creating a loop for testing        list.head.next.next.next.next.next = list.head.next.next;        list.detectAndRemoveLoop(list.head);        Console.WriteLine("Linked List after removing loop : ");        list.printList(list.head);    }} // This code contributed by Rajput-Ji
<script> // Javascript program to detect and// remove loop in linked listvar head; class Node{    constructor(val)    {        this.data = val;        this.next = null;    }} // Function that detects loop in the listfunction detectAndRemoveLoop(node){    var slow = node, fast = node;    while (slow != null &&           fast != null &&           fast.next != null)    {        slow = slow.next;        fast = fast.next.next;         // If slow and fast meet at same        // point then loop is present        if (slow == fast)        {            removeLoop(slow, node);            return 1;        }    }    return 0;} // Function to remove loopfunction removeLoop(loop, head){    var ptr1 = loop;    var ptr2 = loop;     // Count the number of nodes in loop    var k = 1, i;         while (ptr1.next != ptr2)    {        ptr1 = ptr1.next;        k++;    }     // Fix one pointer to head    ptr1 = head;     // And the other pointer to    // k nodes after head    ptr2 = head;    for(i = 0; i < k; i++)    {        ptr2 = ptr2.next;    }     /*  Move both pointers at the same pace,     they will meet at loop starting node */    while (ptr2 != ptr1)    {        ptr1 = ptr1.next;        ptr2 = ptr2.next;    }     // Get pointer to the last node    while (ptr2.next != ptr1)    {        ptr2 = ptr2.next;    }     /* Set the next node of the loop ending node     to fix the loop */    ptr2.next = null;} // Function to print the linked listfunction printList(node){    while (node != null)    {        document.write(node.data + " ");        node = node.next;    }} // Driver codehead = new Node(50);head.next = new Node(20);head.next.next = new Node(15);head.next.next.next = new Node(4);head.next.next.next.next = new Node(10); // Creating a loop for testinghead.next.next.next.next.next = head.next.next;detectAndRemoveLoop(head);document.write("Linked List after removing loop : ");printList(head); // This code is contributed by todaysgaurav </script>
Output: 
Linked List after removing loop 
50 20 15 4 10 
Method 3 (Optimized Method 2: Without Counting Nodes in Loop) We do not need to count number of nodes in Loop. After detecting the loop, if we start slow pointer from head and move both slow and fast pointers at same speed until fast don’t meet, they would meet at the beginning of the loop.
How does this work? Let slow and fast meet at some point after Floyd’s Cycle finding algorithm. Below diagram shows the situation when cycle is found.
We can conclude below from above diagram 
Distance traveled by fast pointer = 2 * (Distance traveled 
                                         by slow pointer)

(m + n*x + k) = 2*(m + n*y + k)

Note that before meeting the point shown above, fast
was moving at twice speed.

x -->  Number of complete cyclic rounds made by 
       fast pointer before they meet first time

y -->  Number of complete cyclic rounds made by 
       slow pointer before they meet first time
From above equation, we can conclude below 
    m + k = (x-2y)*n

Which means m+k is a multiple of n. 
Thus we can write, m + k = i*n or m = i*n - k.
Hence, distance moved by slow pointer: m, is equal to distance moved by fast pointer:
i*n - k or (i-1)*n + n - k (cover the loop completely i-1 times and start from n-k).
So if we start moving both pointers again at same speed such that one pointer (say slow) begins from head node of linked list and other pointer (say fast) begins from meeting point. When slow pointer reaches beginning of loop (has made m steps), fast pointer would have made also moved m steps as they are now moving same pace. Since m+k is a multiple of n and fast starts from k, they would meet at the beginning. Can they meet before also? No because slow pointer enters the cycle first time after m steps. 
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// C++ program to detect and remove loop#include <bits/stdc++.h>using namespace std; struct Node {    int key;    struct Node* next;}; Node* newNode(int key){    Node* temp = new Node;    temp->key = key;    temp->next = NULL;    return temp;} // A utility function to print a linked listvoid printList(Node* head){    while (head != NULL) {        cout << head->key << " ";        head = head->next;    }    cout << endl;} // Function to detect and remove loop in a linked list that// may contain loopvoid detectAndRemoveLoop(Node* head){    // If list is empty or has only one node without loop    if (head == NULL || head->next == NULL)        return;     Node *slow = head, *fast = head;     // Move slow and fast 1 and 2 steps ahead respectively.    slow = slow->next;    fast = fast->next->next;     // Search for loop using slow and fast pointers    while (fast && fast->next) {        if (slow == fast)            break;        slow = slow->next;        fast = fast->next->next;    }     /* If loop exists */    if (slow == fast) {        slow = head;         // this check is needed when slow and fast both meet        // at the head of the LL eg: 1->2->3->4->5 and then        // 5->next = 1 i.e the head of the LL        if (slow == fast)            while (fast->next != slow)                fast = fast->next;        else {            while (slow->next != fast->next) {                slow = slow->next;                fast = fast->next;            }        }         /* since fast->next is the looping point */        fast->next = NULL; /* remove loop */    }} /* Driver program to test above function*/int main(){    Node* head = newNode(50);    head->next = head;    head->next = newNode(20);    head->next->next = newNode(15);    head->next->next->next = newNode(4);    head->next->next->next->next = newNode(10);     /* Create a loop for testing */    head->next->next->next->next->next = head;     detectAndRemoveLoop(head);     printf("Linked List after removing loop \n");    printList(head);     return 0;} // This code is contributed by Aditya Kumar (adityakumar129)
// C++ program to detect and remove loop#include <stdio.h>#include <stdlib.h> typedef struct Node {    int key;    struct Node* next;} Node; Node* newNode(int key){    Node* temp = (Node*)malloc(sizeof(Node));    temp->key = key;    temp->next = NULL;    return temp;} // A utility function to print a linked listvoid printList(Node* head){    while (head != NULL) {        printf("%d  ", head->key);        head = head->next;    }    printf("\n");} // Function to detect and remove loop in a linked list that// may contain loopvoid detectAndRemoveLoop(Node* head){    // If list is empty or has only one node without loop    if (head == NULL || head->next == NULL)        return;     Node *slow = head, *fast = head;     // Move slow and fast 1 and 2 steps ahead respectively.    slow = slow->next;    fast = fast->next->next;     // Search for loop using slow and fast pointers    while (fast && fast->next) {        if (slow == fast)            break;        slow = slow->next;        fast = fast->next->next;    }     /* If loop exists */    if (slow == fast) {        slow = head;         // this check is needed when slow and fast both meet        // at the head of the LL eg: 1->2->3->4->5 and then        // 5->next = 1 i.e the head of the LL        if (slow == fast)            while (fast->next != slow)                fast = fast->next;        else {            while (slow->next != fast->next) {                slow = slow->next;                fast = fast->next;            }        }         /* since fast->next is the looping point */        fast->next = NULL; /* remove loop */    }} /* Driver program to test above function*/int main(){    Node* head = newNode(50);    head->next = head;    head->next = newNode(20);    head->next->next = newNode(15);    head->next->next->next = newNode(4);    head->next->next->next->next = newNode(10);     /* Create a loop for testing */    head->next->next->next->next->next = head;     detectAndRemoveLoop(head);     printf("Linked List after removing loop \n");    printList(head);     return 0;} // This code is contributed by Aditya Kumar (adityakumar129)
// Java program to detect// and remove loop in linked list class LinkedList {     static Node head;     static class Node {         int data;        Node next;         Node(int d)        {            data = d;            next = null;        }    }     // Function that detects loop in the list    void detectAndRemoveLoop(Node node)    {         // If list is empty or has only one node        // without loop        if (node == null || node.next == null)            return;         Node slow = node, fast = node;         // Move slow and fast 1 and 2 steps        // ahead respectively.        slow = slow.next;        fast = fast.next.next;         // Search for loop using slow and fast pointers        while (fast != null && fast.next != null) {            if (slow == fast)                break;             slow = slow.next;            fast = fast.next.next;        }          /* If loop exists */        if (slow == fast) {            slow = node;            if (slow != fast) {                while (slow.next != fast.next) {                    slow = slow.next;                    fast = fast.next;                }                /* since fast->next is the looping point */                fast.next = null; /* remove loop */            }              /* This case is added if fast and slow pointer meet at first position. */            else {                while(fast.next != slow) {                    fast = fast.next;                }                fast.next = null;            }        }    }     // Function to print the linked list    void printList(Node node)    {        while (node != null) {            System.out.print(node.data + " ");            node = node.next;        }    }     // Driver code    public static void main(String[] args)    {        LinkedList list = new LinkedList();        list.head = new Node(50);        list.head.next = new Node(20);        list.head.next.next = new Node(15);        list.head.next.next.next = new Node(4);        list.head.next.next.next.next = new Node(10);         // Creating a loop for testing        head.next.next.next.next.next = head.next.next;        list.detectAndRemoveLoop(head);        System.out.println("Linked List after removing loop : ");        list.printList(head);    }} // This code has been contributed by Mayank Jaiswal
# Python program to detect and remove loop # Node class  class Node:     # Constructor to initialize the node object    def __init__(self, data):        self.data = data        self.next = None  class LinkedList:     # Function to initialize head    def __init__(self):        self.head = None     # Function to insert a new node at the beginning    def push(self, new_data):        new_node = Node(new_data)        new_node.next = self.head        self.head = new_node     def detectAndRemoveLoop(self):       if self.head is None:          return      if self.head.next is None:          return      slow_p = self.head      fast_p = self.head                while(slow_p and fast_p and fast_p.next):          slow_p = slow_p.next          fast_p = fast_p.next.next           # If slow_p and fast_p meet at some point then          # there is a loop          if slow_p == fast_p:            slow_p = self.head              # Finding the beginning of the loop            while (slow_p.next != fast_p.next):              slow_p = slow_p.next              fast_p = fast_p.next                 # Sinc fast.next is the looping point            fast_p.next = None  # Remove loop     # Utility function to print the LinkedList     def printList(self):        temp = self.head        while(temp):            print(temp.data, end = ' ')            temp = temp.next  # Driver programllist = LinkedList()llist.head = Node(50)llist.head.next = Node(20)llist.head.next.next = Node(15)llist.head.next.next.next = Node(4)llist.head.next.next.next.next = Node(10) # Create a loop for testingllist.head.next.next.next.next.next = llist.head.next.next llist.detectAndRemoveLoop() print("Linked List after removing loop")llist.printList() # This code is contributed by Nikhil Kumar Singh(nickzuck_007)
// C# program to detect and remove loop in linked listusing System; public class LinkedList {     public Node head;     public class Node {         public int data;        public Node next;         public Node(int d)        {            data = d;            next = null;        }    }     // Function that detects loop in the list    void detectAndRemoveLoop(Node node)    {         // If list is empty or has only one node        // without loop        if (node == null || node.next == null)            return;         Node slow = node, fast = node;         // Move slow and fast 1 and 2 steps        // ahead respectively.        slow = slow.next;        fast = fast.next.next;         // Search for loop using slow and fast pointers        while (fast != null && fast.next != null) {            if (slow == fast)                break;             slow = slow.next;            fast = fast.next.next;        }         /* If loop exists */        if (slow == fast) {            slow = node;            while (slow.next != fast.next) {                slow = slow.next;                fast = fast.next;            }             /* since fast->next is the looping point */            fast.next = null; /* remove loop */        }    }     // Function to print the linked list    void printList(Node node)    {        while (node != null) {            Console.Write(node.data + " ");            node = node.next;        }    }     // Driver program to test above functions    public static void Main(String[] args)    {        LinkedList list = new LinkedList();        list.head = new Node(50);        list.head.next = new Node(20);        list.head.next.next = new Node(15);        list.head.next.next.next = new Node(4);        list.head.next.next.next.next = new Node(10);         // Creating a loop for testing        list.head.next.next.next.next.next = list.head.next.next;        list.detectAndRemoveLoop(list.head);        Console.WriteLine("Linked List after removing loop : ");        list.printList(list.head);    }} // This code contributed by Rajput-Ji
<script>// javascript program to detect// and remove loop in linked listvar head;      class Node     {            constructor(val)            {                this.data = val;                this.next = null;            }        }     // Function that detects loop in the list    function detectAndRemoveLoop(node) {         // If list is empty or has only one node        // without loop        if (node == null || node.next == null)            return;         var slow = node, fast = node;         // Move slow and fast 1 and 2 steps        // ahead respectively.        slow = slow.next;        fast = fast.next.next;         // Search for loop using slow and fast pointers        while (fast != null && fast.next != null) {            if (slow == fast)                break;             slow = slow.next;            fast = fast.next.next;        }         /* If loop exists */        if (slow == fast) {            slow = node;            if (slow != fast) {                while (slow.next != fast.next) {                    slow = slow.next;                    fast = fast.next;                }                                 /* since fast->next is the looping point */                fast.next = null; /* remove loop */            }                         /* This case is added if fast and            slow pointer meet at first position. */            else {                while (fast.next != slow) {                    fast = fast.next;                }                fast.next = null;            }        }    }     // Function to print the linked list    function printList(node) {        while (node != null) {            document.write(node.data + " ");            node = node.next;        }    }     // Driver code        head = new Node(50);        head.next = new Node(20);        head.next.next = new Node(15);        head.next.next.next = new Node(4);        head.next.next.next.next = new Node(10);         // Creating a loop for testing        head.next.next.next.next.next = head.next.next;        detectAndRemoveLoop(head);        document.write("Linked List after removing loop : ");        printList(head); // This code is contributed by umadevi9616</script>
Output:
Linked List after removing loop 
50 20 15 4 10 
Method 4 Hashing: Hash the address of the linked list nodes We can hash the addresses of the linked list nodes in an unordered map and just check if the element already exists in the map. If it exists, we have reached a node which already exists by a cycle, hence we need to make the last node’s next pointer NULL.
C++
Java
C#
Javascript
// C++ program to detect and remove loop#include <bits/stdc++.h>using namespace std; struct Node {    int key;    struct Node* next;}; Node* newNode(int key){    Node* temp = new Node;    temp->key = key;    temp->next = NULL;    return temp;} // A utility function to print a linked listvoid printList(Node* head){    while (head != NULL) {        cout << head->key << " ";        head = head->next;    }    cout << endl;} // Function to detect and remove loop// in a linked list that may contain loopvoid hashAndRemove(Node* head){    // hash map to hash addresses of the linked list nodes    unordered_map<Node*, int> node_map;    // pointer to last node    Node* last = NULL;    while (head != NULL) {        // if node not present in the map, insert it in the map        if (node_map.find(head) == node_map.end()) {            node_map[head]++;            last = head;            head = head->next;        }        // if present, it is a cycle, make the last node's next pointer NULL        else {            last->next = NULL;            break;        }    }}/* Driver program to test above function*/int main(){    Node* head = newNode(50);    head->next = head;    head->next = newNode(20);    head->next->next = newNode(15);    head->next->next->next = newNode(4);    head->next->next->next->next = newNode(10);     /* Create a loop for testing */    head->next->next->next->next->next = head->next->next;     // printList(head);    hashAndRemove(head);     printf("Linked List after removing loop \n");    printList(head);     return 0;}
// Java program to detect  and remove loop in a linked listimport java.util.*; public class LinkedList {     static Node head; // head of list     /* Linked list Node*/    static class Node {        int data;        Node next;        Node(int d)        {            data = d;            next = null;        }    }     /* Inserts a new Node at front of the list. */    static public void push(int new_data)    {        /* 1 & 2: Allocate the Node &                Put in the data*/        Node new_node = new Node(new_data);         /* 3. Make next of new Node as head */        new_node.next = head;         /* 4. Move the head to point to new Node */        head = new_node;    }     // Function to print the linked list    void printList(Node node)    {        while (node != null) {            System.out.print(node.data + " ");            node = node.next;        }    }     // Returns true if the loop is removed from the linked    // list else returns false.    static boolean removeLoop(Node h)    {        HashSet<Node> s = new HashSet<Node>();        Node prev = null;        while (h != null) {            // If we have already has this node            // in hashmap it means their is a cycle and we            // need to remove this cycle so set the next of            // the previous pointer with null.             if (s.contains(h)) {                prev.next = null;                return true;            }             // If we are seeing the node for            // the first time, insert it in hash            else {                s.add(h);                prev = h;                h = h.next;            }        }         return false;    }     /* Driver program to test above function */    public static void main(String[] args)    {        LinkedList llist = new LinkedList();         llist.push(20);        llist.push(4);        llist.push(15);        llist.push(10);         /*Create loop for testing */        llist.head.next.next.next.next = llist.head;         if (removeLoop(head)) {            System.out.println("Linked List after removing loop");            llist.printList(head);        }        else            System.out.println("No Loop found");    }} // This code is contributed by Animesh Nag.
// C# program to detect  and remove loop in a linked list using System;using System.Collections.Generic;public class LinkedList {     public Node head; // head of list     /* Linked list Node*/    public class Node {        public int data;        public Node next;        public Node(int d)        {            data = d;            next = null;        }    }        // Function to print the linked list    void printList(Node node)    {        while (node != null) {            Console.Write(node.data + " ");            node = node.next;        }    }     // Returns true if the loop is removed from the linked    // list else returns false.    bool removeLoop(Node h)    {        HashSet<Node> s = new HashSet<Node>();        Node prev = null;        while (h != null) {            // If we have already has this node            // in hashmap it means their is a cycle and we            // need to remove this cycle so set the next of            // the previous pointer with null.             if (s.Contains(h)) {                prev.next = null;                return true;            }             // If we are seeing the node for            // the first time, insert it in hash            else {                s.Add(h);                prev = h;                h = h.next;            }        }         return false;    }     /* Driver program to test above function */    public static void Main()    {                LinkedList list = new LinkedList();        list.head = new Node(50);        list.head.next = new Node(20);        list.head.next.next = new Node(15);        list.head.next.next.next = new Node(4);        list.head.next.next.next.next = new Node(10);         /*Create loop for testing */        list.head.next.next.next.next.next = list.head.next.next;         if (list.removeLoop(list.head)) {            Console.WriteLine("Linked List after removing loop");            list.printList(list.head);        }        else            Console.WriteLine("No Loop found");    }} // This code is contributed by ihritik
<script>// javascript program to detect  and remove loop in a linked list class LinkedList {     /* Linked list Node */     class Node {            constructor(val) {                this.data = val;                this.next = null;            }        }     var head; // head of list     /* Inserts a new Node at front of the list. */      function push(new_data) {        /*         * 1 & 2: Allocate the Node & Put in the data         */var new_node = new Node(new_data);         /* 3. Make next of new Node as head */        new_node.next = head;         /* 4. Move the head to point to new Node */        head = new_node;        return head;    }     // Function to print the linked list    function printList(node) {        while (node != null) {            document.write(node.data + " ");            node = node.next;        }    }     // Returns true if the loop is removed from the linked    // list else returns false.    function removeLoop(h)    {             var s = new Set();        var prev = null;        while (h != null)        {                     // If we have already has this node            // in hashmap it means their is a cycle and we            // need to remove this cycle so set the next of            // the previous pointer with null.             if (s.has(h)) {                prev.next = null;                return true;            }             // If we are seeing the node for            // the first time, insert it in hash            else {                s.add(h);                prev = h;                h = h.next;            }        }         return false;    }     /* Driver program to test above function */        head = push(50);        head.next = push(20);        head.next.next=push(4);        head.next.next.next=push(15);        head.next.next.next.next=push(10);         /* Create loop for testing */        head.next.next.next.next.next = head.next.next;         if (removeLoop(head)) {            document.write("Linked List after removing loop<br/>");            printList(head);        } else            document.write("No Loop found"); // This code is contributed by gauravrajput1</script>
Linked List after removing loop 
50 20 15 4 10 
We Thank Shubham Agrawal for suggesting this solution.
Thanks to Gaurav Ahirwar for suggesting the above solution.Please write comments if you find the above codes/algorithms incorrect, or find other ways to solve the same problem. 
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