Make middle node head in a linked list

22 Jun, 2022
Given a singly linked list, find middle of the linked list and set middle node of the linked list at beginning of the linked list. Examples:  
Input  : 1 2 3 4 5 
Output : 3 1 2 4 5

Input  : 1 2 3 4 5 6
Output : 4 1 2 3 5 6 
 
 
The idea is to first find middle of a linked list using two pointers, first one moves one at a time and second one moves two at a time. When second pointer reaches end, first reaches middle. We also keep track of previous of first pointer so that we can remove middle node from its current position and can make it head.  
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// C++ program to make middle node as head of // linked list. #include <bits/stdc++.h>using namespace std;  /* Link list node */class Node {     public:    int data;     Node* next; };   /* Function to get the middle and set at beginning of the linked list*/void setMiddleHead(Node** head) {     if (*head == NULL)         return;       // To traverse list nodes one by one     Node* one_node = (*head);       // To traverse list nodes by skipping     // one.     Node* two_node = (*head);       // To keep track of previous of middle     Node* prev = NULL;     while (two_node != NULL && two_node->next != NULL)     {           /* for previous node of middle node */        prev = one_node;           /* move one node each time*/        two_node = two_node->next->next;           /* move two node each time*/        one_node = one_node->next;     }       /* set middle node at head */    prev->next = prev->next->next;     one_node->next = (*head);     (*head) = one_node; }   // To insert a node at the beginning of linked // list. void push(Node** head_ref, int new_data) {     /* allocate node */    Node* new_node = new Node();       new_node->data = new_data;       /* link the old list off the new node */    new_node->next = (*head_ref);       /* move the head to point to the new node */    (*head_ref) = new_node; }   // A function to print a given linked list void printList(Node* ptr) {     while (ptr != NULL)    {         cout << ptr->data << " ";         ptr = ptr->next;     }     cout<<endl; }   /* Driver code*/int main() {     // Create a list of 5 nodes     Node* head = NULL;     int i;     for (i = 5; i > 0; i--)         push(&head, i);       cout << " list before: ";     printList(head);       setMiddleHead(&head);       cout << " list After: ";     printList(head);       return 0; }   // This is code is contributed by rathbhupendra
// C program to make middle node as head of// linked list.#include <stdio.h>#include <stdlib.h>  /* Link list node */struct Node {    int data;    struct Node* next;};  /* Function to get the middle and set at   beginning of the linked list*/void setMiddleHead(struct Node** head){    if (*head == NULL)        return;      // To traverse list nodes one by one    struct Node* one_node = (*head);      // To traverse list nodes by skipping    // one.    struct Node* two_node = (*head);      // To keep track of previous of middle    struct Node* prev = NULL;    while (two_node != NULL && two_node->next != NULL) {          /* for previous node of middle node */        prev = one_node;          /* move one node each time*/        two_node = two_node->next->next;          /* move two node each time*/        one_node = one_node->next;    }      /* set middle node at head */    prev->next = prev->next->next;    one_node->next = (*head);    (*head) = one_node;}  // To insert a node at the beginning of linked// list.void push(struct Node** head_ref, int new_data){    /* allocate node */    struct Node* new_node =        (struct Node*)malloc(sizeof(struct Node));      new_node->data = new_data;      /* link the old list off the new node */    new_node->next = (*head_ref);      /* move the head to point to the new node */    (*head_ref) = new_node;}  // A  function to print a given linked listvoid printList(struct Node* ptr){    while (ptr != NULL) {        printf("%d ", ptr->data);        ptr = ptr->next;    }    printf("\n");}  /* Driver function*/int main(){    // Create a list of 5 nodes    struct Node* head = NULL;    int i;    for (i = 5; i > 0; i--)        push(&head, i);      printf(" list before: ");    printList(head);      setMiddleHead(&head);      printf(" list After:  ");    printList(head);      return 0;}
// Java program to make middle node // as head of Linked listpublic class GFG {         /* Link list node */    static class Node {        int data;        Node next;        Node(int data){            this.data = data;            next = null;        }    }          static Node head;          /* Function to get the middle and     set at beginning of the linked list*/    static void setMiddleHead()    {        if (head == null)            return;               // To traverse list nodes one         // by one        Node one_node = head;               // To traverse list nodes by         // skipping one.        Node two_node = head;               // To keep track of previous of middle        Node prev = null;        while (two_node != null &&                two_node.next != null) {                   /* for previous node of middle node */            prev = one_node;                   /* move one node each time*/            two_node = two_node.next.next;                   /* move two node each time*/            one_node = one_node.next;        }               /* set middle node at head */        prev.next = prev.next.next;        one_node.next = head;        head = one_node;    }           // To insert a node at the beginning of    // linked list.    static void push(int new_data)    {        /* allocate node */        Node new_node = new Node(new_data);               /* link the old list off the new node */        new_node.next = head;               /* move the head to point to the new node */        head = new_node;    }           // A  function to print a given linked list    static void printList(Node ptr)    {        while (ptr != null) {            System.out.print(ptr.data+" ");            ptr = ptr.next;        }        System.out.println();    }           /* Driver function*/    public static void main(String args[])    {        // Create a list of 5 nodes        head = null;        int i;        for (i = 5; i > 0; i--)            push(i);                  System.out.print(" list before: ");        printList(head);               setMiddleHead();               System.out.print(" list After:  ");        printList(head);          }}// This code is contributed by Sumit Ghosh
# Python3 program to make middle node# as head of Linked list  # Linked List nodeclass Node:    def __init__(self, data):        self.data = data        self.next = None  # function to get the middle node# set it as the beginning of the # linked listdef setMiddleHead(head):    if(head == None):        return None      # to traverse nodes     # one by one    one_node = head      # to traverse nodes by     # skipping one    two_node = head      # to keep track of previous middle    prev = None      while(two_node != None and          two_node.next != None):          # for previous node of middle node        prev = one_node          # move one node each time        one_node = one_node.next          # move two nodes each time        two_node = two_node.next.next      # set middle node at head    prev.next = prev.next.next    one_node.next = head    head = one_node      # return the modified head    return head  def push(head, new_data):      # allocate new node    new_node = Node(new_data)      #link the old list to new node    new_node.next = head      # move the head to point the new node    head = new_node      # return the modified head    return head  # A function to print a given linked listdef printList(head):    temp = head    while (temp!=None):                  print(str(temp.data), end = " ")        temp = temp.next    print("")  # Create a list of 5 nodeshead = Nonefor i in range(5, 0, -1):    head = push(head, i)  print(" list before: ", end = "")printList(head)  head = setMiddleHead(head)  print(" list After: ", end = "")printList(head)  # This code is contributed# by Pranav Devarakonda
// C# program to make middle node // as head of Linked listusing System;  public class GFG {         /* Link list node */    class Node     {        public int data;        public Node next;        public Node(int data)        {            this.data = data;            next = null;        }    }          static Node head;          /* Function to get the middle and     set at beginning of the linked list*/    static void setMiddleHead()    {        if (head == null)            return;              // To traverse list nodes one         // by one        Node one_node = head;              // To traverse list nodes by         // skipping one.        Node two_node = head;              // To keep track of previous of middle        Node prev = null;        while (two_node != null &&             two_node.next != null)         {                  /* for previous node of middle node */            prev = one_node;                  /* move one node each time*/            two_node = two_node.next.next;                  /* move two node each time*/            one_node = one_node.next;        }              /* set middle node at head */        prev.next = prev.next.next;        one_node.next = head;        head = one_node;    }          // To insert a node at the beginning of    // linked list.    static void push(int new_data)    {        /* allocate node */        Node new_node = new Node(new_data);              /* link the old list off the new node */        new_node.next = head;              /* move the head to point to the new node */        head = new_node;    }          // A function to print a given linked list    static void printList(Node ptr)    {        while (ptr != null) {            Console.Write(ptr.data + " ");            ptr = ptr.next;        }        Console.WriteLine();    }          /* Driver code*/    public static void Main(String []args)    {        // Create a list of 5 nodes        head = null;        int i;        for (i = 5; i > 0; i--)            push(i);                  Console.Write(" list before: ");        printList(head);              setMiddleHead();              Console.Write(" list After: ");        printList(head);    }}  // This code is contributed by Rajput-Ji
<script>// javascript program to make middle node // as head of Linked list      /* Link list node */class Node {    constructor(val) {        this.data = val;        this.next = null;    }}      var head;      /*     * Function to get the middle and set at beginning of the linked list     */    function setMiddleHead() {        if (head == null)            return;          // To traverse list nodes one        // by one         one_node = head;          // To traverse list nodes by        // skipping one.         two_node = head;          // To keep track of previous of middle         prev = null;        while (two_node != null && two_node.next != null) {              /* for previous node of middle node */            prev = one_node;              /* move one node each time */            two_node = two_node.next.next;              /* move two node each time */            one_node = one_node.next;        }          /* set middle node at head */        prev.next = prev.next.next;        one_node.next = head;        head = one_node;    }      // To insert a node at the beginning of    // linked list.    function push(new_data)    {              /* allocate node */         new_node = new Node(new_data);          /* link the old list off the new node */        new_node.next = head;          /* move the head to point to the new node */        head = new_node;    }      // A function to print a given linked list    function printList( ptr) {        while (ptr != null) {            document.write(ptr.data + " ");            ptr = ptr.next;        }        document.write("<br/>");    }      /* Driver function */              // Create a list of 5 nodes        head = null;        var i;        for (i = 5; i > 0; i--)            push(i);          document.write(" list before: ");        printList(head);          setMiddleHead();          document.write(" list After:  ");        printList(head);  // This code is contributed by aashish1995 </script>
Output:  
list before: 1 2 3 4 5
list After : 3 1 2 4 5 
Time Complexity : O(n) Auxiliary Space : O(1) 
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