Make middle node head in a linked list

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. 

Implementation:

C++

// 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 of 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

// 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 of 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(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

// Java program to make middle node
// as head of Linked list
public 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 of 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

# Python3 program to make middle node
# as head of Linked list

# Linked List node
class 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 list
def 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 list
def printList(head):
    temp = head
    while (temp!=None):

        print(str(temp.data), end = " ")
        temp = temp.next
    print("")

# Create a list of 5 nodes
head = None
for 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#

// C# program to make middle node
// as head of Linked list
using 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 of 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

Javascript

<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 of 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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