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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 second list into first list at alternate positions of 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 second list should only be inserted when there are positions available. For example, if the first list is 1->2->3 and second list is 4->5->6->7->8, then first list should become 1->4->2->5->3->6 and 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. Expected time complexity is O(n) where n is number of nodes in 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 <bits/stdc++.h>
using namespace std;
// A nexted list node
class Node
    int data;
    Node *next;
/* Function to insert a node
   at the beginning */
void push(Node ** head_ref,
          int new_data)
    Node* new_node = new 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(Node *head)
    Node *temp = head;
    while (temp != NULL)
        cout << temp -> data << " ";
        temp = temp -> next;
    cout << endl;
// 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(Node *p, Node **q)
    Node *p_curr = p, *q_curr = *q;
    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()
    Node *p = NULL, *q = NULL;
    push(&p, 3);
    push(&p, 2);
    push(&p, 1);
    cout << "First Linked List:";
    push(&q, 8);
    push(&q, 7);
    push(&q, 6);
    push(&q, 5);
    push(&q, 4);
    cout << "Second Linked List:";
    merge(p, &q);
    cout <<
    "Modified First Linked List:";
    cout <<
    "Modified Second Linked List:";
    return 0;
// This code is contributed by rathbhupendra


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(min(n1, n2)), where n1 and n2  represents the length of the given two linked lists.
Auxiliary Space: O(1), no extra space is required, so it is a constant.

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

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Last Updated : 16 Jun, 2022
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