Javascript Program For Making 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.

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) when n is the total no of nodes in a linked list

Space Complexity: O(1) because using constant space

Please refer complete article on Make middle node head in a linked list for more details!