C Program To Merge A Linked List Into Another Linked List At Alternate Positions

// C program to merge a linked list into 
// another at alternate positions
#include <stdio.h>
#include <stdlib.h>

// A nested list node
struct Node
{
    int data;
    struct Node *next;
};

/* Function to insert a node at 
   the beginning */
void push(struct Node ** head_ref, 
          int new_data)
{
    struct Node* new_node = 
           (struct Node*) malloc(sizeof(struct Node));
    new_node->data  = new_data;
    new_node->next = (*head_ref);
    (*head_ref)  = new_node;
}

/* Utility function to print a 
   singly linked list */
void printList(struct Node *head)
{
    struct Node *temp = head;
    while (temp != NULL)
    {
        printf("%d ", temp->data);
        temp = temp->next;
    }
    printf("");
}

// Main function that inserts nodes of 
// linked list q into p at alternate 
// positions. Since head of first list 
// never changes and head of second list  
// may change, we need single pointer
// for first list and double pointer for 
// second list.
void merge(struct Node *p, struct Node **q)
{
     struct Node *p_curr = p, 
                 *q_curr = *q;
     struct Node *p_next, *q_next;

     // While there are available 
     // positions in p
     while (p_curr != NULL && 
            q_curr != NULL)
     {
         // Save next pointers
         p_next = p_curr->next;
         q_next = q_curr->next;

         // Make q_curr as next of p_curr
         // Change next pointer of q_curr
         q_curr->next = p_next;  

         // Change next pointer of p_curr
         p_curr->next = q_curr;  

         // Update current pointers for 
         // next iteration
         p_curr = p_next;
         q_curr = q_next;
    }

    // Update head pointer of second list
    *q = q_curr; 
}

// Driver code
int main()
{
     struct Node *p = NULL, *q = NULL;
     push(&p, 3);
     push(&p, 2);
     push(&p, 1);
     printf(
     "First Linked List:");
     printList(p);

     push(&q, 8);
     push(&q, 7);
     push(&q, 6);
     push(&q, 5);
     push(&q, 4);
     printf(
     "Second Linked List:");
     printList(q);

     merge(p, &q);

     printf(
     "Modified First Linked List:");
     printList(p);

     printf(
     "Modified Second Linked List:");
     printList(q);

     getchar();
     return 0;
}