C 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. 

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("");
}

// Driver code
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;
}

Output:

list before: 1 2 3 4 5
list After : 3 1 2 4 5 

Time Complexity: O(n) where n is the total number of nodes in the linked list.

Auxiliary Space: O(1) since using constant space

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