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

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Given two linked lists, insert nodes of the second list into the first list at alternate positions of the first list. 
For example, if first list is 5->7->17->13->11 and second is 12->10->2->4->6, the first list should become 5->12->7->10->17->2->13->4->11->6 and second list should become empty. The nodes of the second list should only be inserted when there are positions available. For example, if the first list is 1->2->3 and the second list is 4->5->6->7->8, then the first list should become 1->4->2->5->3->6 and the second list to 7->8.
Use of extra space is not allowed (Not allowed to create additional nodes), i.e., insertion must be done in-place. The expected time complexity is O(n) where n is a number of nodes in the first list. 

The idea is to run a loop while there are available positions in first loop and insert nodes of second list by changing pointers. Following are implementations of this approach. 


// 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;
// 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);
     "First Linked List:");
     push(&q, 8);
     push(&q, 7);
     push(&q, 6);
     push(&q, 5);
     push(&q, 4);
     "Second Linked List:");
     merge(p, &q);
     "Modified First Linked List:");
     "Modified Second Linked List:");
     return 0;


First Linked List:
1 2 3
Second Linked List:
4 5 6 7 8
Modified First Linked List:
1 4 2 5 3 6
Modified Second Linked List:
7 8 

Time Complexity: O(N)

Auxiliary Space: O(1)

Please refer complete article on Merge a linked list into another linked list at alternate positions for more details!

Last Updated : 31 Mar, 2022
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