Remove duplicates from an unsorted linked list

14 Jun, 2022
Write a removeDuplicates() function that takes a list and deletes any duplicate nodes from the list. The list is not sorted. For example if the linked list is 12->11->12->21->41->43->21 then removeDuplicates() should convert the list to 12->11->21->41->43.  
METHOD 1 (Using two loops) This is the simple way where two loops are used. Outer loop is used to pick the elements one by one and the inner loop compares the picked element with the rest of the elements. Thanks to Gaurav Saxena for his help in writing this code.
C++
Java
Python3
C#
Javascript
/* C++ Program to remove duplicates in an unsorted   linked list */#include <bits/stdc++.h>using namespace std; /* A linked list node */struct Node {    int data;    struct Node* next;}; // Utility function to create a new Nodestruct Node* newNode(int data){    Node* temp = new Node;    temp->data = data;    temp->next = NULL;    return temp;} /* Function to remove duplicates from a   unsorted linked list */void removeDuplicates(struct Node* start){    struct Node *ptr1, *ptr2, *dup;    ptr1 = start;     /* Pick elements one by one */    while (ptr1 != NULL && ptr1->next != NULL) {        ptr2 = ptr1;         /* Compare the picked element with rest           of the elements */        while (ptr2->next != NULL) {            /* If duplicate then delete it */            if (ptr1->data == ptr2->next->data) {                /* sequence of steps is important here */                dup = ptr2->next;                ptr2->next = ptr2->next->next;                delete (dup);            }            else /* This is tricky */                ptr2 = ptr2->next;        }        ptr1 = ptr1->next;    }} /* Function to print nodes in a given linked list */void printList(struct Node* node){    while (node != NULL) {        printf("%d ", node->data);        node = node->next;    }} /* Driver program to test above function */int main(){    /* The constructed linked list is:     10->12->11->11->12->11->10*/    struct Node* start = newNode(10);    start->next = newNode(12);    start->next->next = newNode(11);    start->next->next->next = newNode(11);    start->next->next->next->next = newNode(12);    start->next->next->next->next->next = newNode(11);    start->next->next->next->next->next->next = newNode(10);     printf("Linked list before removing duplicates ");    printList(start);     removeDuplicates(start);     printf("\nLinked list after removing duplicates ");    printList(start);     return 0;}
// Java program to remove duplicates from unsorted// linked list class LinkedList {     static Node head;     static class Node {         int data;        Node next;         Node(int d)        {            data = d;            next = null;        }    }     /* Function to remove duplicates from an       unsorted linked list */    void remove_duplicates()    {        Node ptr1 = null, ptr2 = null, dup = null;        ptr1 = head;         /* Pick elements one by one */        while (ptr1 != null && ptr1.next != null) {            ptr2 = ptr1;             /* Compare the picked element with rest                of the elements */            while (ptr2.next != null) {                 /* If duplicate then delete it */                if (ptr1.data == ptr2.next.data) {                     /* sequence of steps is important here                     */                    ptr2.next = ptr2.next.next;                    System.gc();                }                else /* This is tricky */ {                    ptr2 = ptr2.next;                }            }            ptr1 = ptr1.next;        }    }     void printList(Node node)    {        while (node != null) {            System.out.print(node.data + " ");            node = node.next;        }    }     public static void main(String[] args)    {        LinkedList list = new LinkedList();        list.head = new Node(10);        list.head.next = new Node(12);        list.head.next.next = new Node(11);        list.head.next.next.next = new Node(11);        list.head.next.next.next.next = new Node(12);        list.head.next.next.next.next.next = new Node(11);        list.head.next.next.next.next.next.next            = new Node(10);         System.out.println(            "Linked List before removing duplicates : \n ");        list.printList(head);         list.remove_duplicates();        System.out.println("");        System.out.println(            "Linked List after removing duplicates : \n ");        list.printList(head);    }}// This code has been contributed by Mayank Jaiswal
# Python3 program to remove duplicates# from unsorted linked list  class Node():     def __init__(self, data):         self.data = data        self.next = None  class LinkedList():     def __init__(self):         # Head of list        self.head = None     def remove_duplicates(self):         ptr1 = None        ptr2 = None        dup = None        ptr1 = self.head         # Pick elements one by one        while (ptr1 != None and ptr1.next != None):             ptr2 = ptr1             # Compare the picked element with rest            # of the elements            while (ptr2.next != None):                 # If duplicate then delete it                if (ptr1.data == ptr2.next.data):                     # Sequence of steps is important here                    dup = ptr2.next                    ptr2.next = ptr2.next.next                else:                    ptr2 = ptr2.next             ptr1 = ptr1.next     # Function to print nodes in a    # given linked list    def printList(self):        temp = self.head         while(temp != None):            print(temp.data, end=" ")            temp = temp.next         print()  # Driver codelist = LinkedList()list.head = Node(10)list.head.next = Node(12)list.head.next.next = Node(11)list.head.next.next.next = Node(11)list.head.next.next.next.next = Node(12)list.head.next.next.next.next.next = Node(11)list.head.next.next.next.next.next.next = Node(10) print("Linked List before removing duplicates :")list.printList()list.remove_duplicates()print()print("Linked List after removing duplicates :")list.printList() # This code is contributed by maheshwaripiyush9
// C# program to remove duplicates from unsorted// linked listusing System;class List_ {     Node head;     class Node {         public int data;        public Node next;         public             Node(int d)        {            data = d;            next = null;        }    }     /* Function to remove duplicates from an         unsorted linked list */    void remove_duplicates()    {        Node ptr1 = null, ptr2 = null, dup = null;        ptr1 = head;         /* Pick elements one by one */        while (ptr1 != null && ptr1.next != null) {            ptr2 = ptr1;             /* Compare the picked element with rest                      of the elements */            while (ptr2.next != null) {                 /* If duplicate then delete it */                if (ptr1.data == ptr2.next.data) {                     /* sequence of steps is important here                     */                    dup = ptr2.next;                    ptr2.next = ptr2.next.next;                }                else /* This is tricky */                {                    ptr2 = ptr2.next;                }            }            ptr1 = ptr1.next;        }    }     void printList(Node node)    {        while (node != null) {            Console.Write(node.data + " ");            node = node.next;        }    }     // Driver Code    public static void Main(String[] args)    {        List_ list = new List_();        list.head = new Node(10);        list.head.next = new Node(12);        list.head.next.next = new Node(11);        list.head.next.next.next = new Node(11);        list.head.next.next.next.next = new Node(12);        list.head.next.next.next.next.next = new Node(11);        list.head.next.next.next.next.next.next            = new Node(10);         Console.WriteLine(            "Linked List_ before removing duplicates : \n ");        list.printList(list.head);         list.remove_duplicates();        Console.WriteLine("");        Console.WriteLine(            "Linked List_ after removing duplicates : \n ");        list.printList(list.head);    }} // This code is contributed by gauravrajput1
<script>// javascript program to remove duplicates from unsorted// linked list       var head;     class Node {        constructor(val) {            this.data = val;            this.next = null;        }    }    /*     * Function to remove duplicates from an unsorted linked list     */    function remove_duplicates() {        var ptr1 = null, ptr2 = null, dup = null;        ptr1 = head;         /* Pick elements one by one */        while (ptr1 != null && ptr1.next != null) {            ptr2 = ptr1;             /*             * Compare the picked element with rest of the elements             */            while (ptr2.next != null) {                 /* If duplicate then delete it */                if (ptr1.data == ptr2.next.data) {                     /* sequence of steps is important here */                    dup = ptr2.next;                    ptr2.next = ptr2.next.next;                     } else /* This is tricky */ {                    ptr2 = ptr2.next;                }            }            ptr1 = ptr1.next;        }    }     function printList( node) {        while (node != null) {            document.write(node.data + " ");            node = node.next;        }    }              head = new Node(10);        head.next = new Node(12);        head.next.next = new Node(11);        head.next.next.next = new Node(11);        head.next.next.next.next = new Node(12);        head.next.next.next.next.next = new Node(11);        head.next.next.next.next.next.next = new Node(10);         document.write("Linked List before removing duplicates : <br/> ");        printList(head);         remove_duplicates();        document.write("<br/>");        document.write("Linked List after removing duplicates : <br/> ");        printList(head); // This code contributed by aashish1995</script>
Linked list before removing duplicates 10 12 11 11 12 11 10 
Linked list after removing duplicates 10 12 11 
Time Complexity: O(n^2)
Auxiliary Space: O(1)
METHOD 2 (Use Sorting) In general, Merge Sort is the best-suited sorting algorithm for sorting linked lists efficiently. 1) Sort the elements using Merge Sort. We will soon be writing a post about sorting a linked list. O(nLogn) 2) Remove duplicates in linear time using the algorithm for removing duplicates in sorted Linked List. O(n) Please note that this method doesn’t preserve the original order of elements.Time Complexity: O(nLogn)
METHOD 3 (Use Hashing) We traverse the link list from head to end. For every newly encountered element, we check whether it is in the hash table: if yes, we remove it; otherwise we put it in the hash table.
C++
Java
Python3
C#
Javascript
/* C++ Program to remove duplicates in an unsorted   linked list */#include <bits/stdc++.h>using namespace std; /* A linked list node */struct Node {    int data;    struct Node* next;}; // Utility function to create a new Nodestruct Node* newNode(int data){    Node* temp = new Node;    temp->data = data;    temp->next = NULL;    return temp;} /* Function to remove duplicates from a   unsorted linked list */void removeDuplicates(struct Node* start){    // Hash to store seen values    unordered_set<int> seen;     /* Pick elements one by one */    struct Node* curr = start;    struct Node* prev = NULL;    while (curr != NULL) {        // If current value is seen before        if (seen.find(curr->data) != seen.end()) {            prev->next = curr->next;            delete (curr);        }        else {            seen.insert(curr->data);            prev = curr;        }        curr = prev->next;    }} /* Function to print nodes in a given linked list */void printList(struct Node* node){    while (node != NULL) {        printf("%d ", node->data);        node = node->next;    }} /* Driver program to test above function */int main(){    /* The constructed linked list is:     10->12->11->11->12->11->10*/    struct Node* start = newNode(10);    start->next = newNode(12);    start->next->next = newNode(11);    start->next->next->next = newNode(11);    start->next->next->next->next = newNode(12);    start->next->next->next->next->next = newNode(11);    start->next->next->next->next->next->next = newNode(10);     printf("Linked list before removing duplicates : \n");    printList(start);     removeDuplicates(start);     printf("\nLinked list after removing duplicates : \n");    printList(start);     return 0;}
// Java program to remove duplicates// from unsorted linkedlist import java.util.HashSet; public class removeDuplicates {    static class node {        int val;        node next;         public node(int val) { this.val = val; }    }     /* Function to remove duplicates from a       unsorted linked list */    static void removeDuplicate(node head)    {        // Hash to store seen values        HashSet<Integer> hs = new HashSet<>();         /* Pick elements one by one */        node current = head;        node prev = null;        while (current != null) {            int curval = current.val;             // If current value is seen before            if (hs.contains(curval)) {                prev.next = current.next;            }            else {                hs.add(curval);                prev = current;            }            current = current.next;        }    }     /* Function to print nodes in a given linked list */    static void printList(node head)    {        while (head != null) {            System.out.print(head.val + " ");            head = head.next;        }    }     public static void main(String[] args)    {        /* The constructed linked list is:         10->12->11->11->12->11->10*/        node start = new node(10);        start.next = new node(12);        start.next.next = new node(11);        start.next.next.next = new node(11);        start.next.next.next.next = new node(12);        start.next.next.next.next.next = new node(11);        start.next.next.next.next.next.next = new node(10);         System.out.println(            "Linked list before removing duplicates :");        printList(start);         removeDuplicate(start);         System.out.println(            "\nLinked list after removing duplicates :");        printList(start);    }} // This code is contributed by Rishabh Mahrsee
# Python3 program to remove duplicates# from unsorted linkedlist  class Node:     def __init__(self, data):         self.data = data        self.next = None  class LinkedList:     def __init__(self):         self.head = None     # Function to print nodes in a    # given linked list    def printlist(self):         temp = self.head         while (temp):            print(temp.data, end=" ")            temp = temp.next     # Function to remove duplicates from a    # unsorted linked list    def removeDuplicates(self, head):         # Base case of empty list or        # list with only one element        if self.head is None or self.head.next is None:            return head         # Hash to store seen values        hash = set()         current = head        hash.add(self.head.data)         while current.next is not None:             if current.next.data in hash:                current.next = current.next.next            else:                hash.add(current.next.data)                current = current.next         return head  # Driver codeif __name__ == "__main__":     # Creating Empty list    llist = LinkedList()    llist.head = Node(10)    second = Node(12)    third = Node(11)    fourth = Node(11)    fifth = Node(12)    sixth = Node(11)    seventh = Node(10)     # Connecting second and third    llist.head.next = second    second.next = third    third.next = fourth    fourth.next = fifth    fifth.next = sixth    sixth.next = seventh     # Printing data    print("Linked List before removing Duplicates.")    llist.printlist()    llist.removeDuplicates(llist.head)    print("\nLinked List after removing duplicates.")    llist.printlist() # This code is contributed by rajataro0
// C# program to remove duplicates// from unsorted linkedlistusing System;using System.Collections.Generic; class removeDuplicates {    class node {        public int val;        public node next;         public node(int val) { this.val = val; }    }     // Function to remove duplicates from a    // unsorted linked list    static void removeDuplicate(node head)    {         // Hash to store seen values        HashSet<int> hs = new HashSet<int>();         // Pick elements one by one        node current = head;        node prev = null;        while (current != null) {            int curval = current.val;             // If current value is seen before            if (hs.Contains(curval)) {                prev.next = current.next;            }            else {                hs.Add(curval);                prev = current;            }            current = current.next;        }    }     // Function to print nodes in a    // given linked list    static void printList(node head)    {        while (head != null) {            Console.Write(head.val + " ");            head = head.next;        }    }     // Driver code    public static void Main(String[] args)    {         // The constructed linked list is:        // 10->12->11->11->12->11->10        node start = new node(10);        start.next = new node(12);        start.next.next = new node(11);        start.next.next.next = new node(11);        start.next.next.next.next = new node(12);        start.next.next.next.next.next = new node(11);        start.next.next.next.next.next.next = new node(10);         Console.WriteLine("Linked list before removing "                          + "duplicates :");        printList(start);         removeDuplicate(start);         Console.WriteLine("\nLinked list after removing "                          + "duplicates :");        printList(start);    }} // This code is contributed by amal kumar choubey
<script> // JavaScript program to remove duplicates// from unsorted linkedlist      class node {         constructor(val) {            this.val = val;            this.next = null;        }    }     /*     Function to remove duplicates     from a unsorted linked list     */    function removeDuplicate( head) {        // Hash to store seen values        var hs =  new Set();         /* Pick elements one by one */        var current = head;        var prev = null;        while (current != null) {            var curval = current.val;             // If current value is seen before            if (hs.has(curval)) {                prev.next = current.next;            } else {                hs.add(curval);                prev = current;            }            current = current.next;        }     }     /* Function to print nodes in a    given linked list */    function printList( head) {        while (head != null) {            document.write(head.val + " ");            head = head.next;        }    }              /*         * The constructed linked list is:         10->12->11->11->12->11->10         */         start = new node(10);        start.next = new node(12);        start.next.next = new node(11);        start.next.next.next = new node(11);        start.next.next.next.next = new node(12);        start.next.next.next.next.next = new node(11);        start.next.next.next.next.next.next = new node(10);         document.write(        "Linked list before removing duplicates :<br/>"        );        printList(start);         removeDuplicate(start);         document.write(        "<br/>Linked list after removing duplicates :<br/>"        );        printList(start); // This code is contributed by todaysgaurav </script>
Linked list before removing duplicates : 
10 12 11 11 12 11 10 
Linked list after removing duplicates : 
10 12 11 
Thanks to bearwang for suggesting this method.Time Complexity: O(N) on average 
(assuming that hash table access time is O(1) on average).  
Auxiliary Space : O(N)
As extra space is used to store the elements in the stack.
–oI4Please write comments if you find any of the above explanations/algorithms incorrect, or a better ways to solve the same problem. 
 
GauravRajput1
Amal Kumar Choubey
rajataro0
simranarora5sos
aashish1995
todaysgaurav
maheswaripiyush9
coder229coder
mohitsinghal63
abhijeet19403
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Merge Sort
Linked List
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Merge Sort
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