Merge a linked list into another at alternate positions

Given two linked lists A and B, we need to insert nodes of B into A at alternative positions of list A.


Time complexity : O(n),

n is number of nodes in first list.


a. Traverse in the ListA until there are available positions in it.
b. Loop in ListB and insert the nodes of ListB to ListA
c. Do insertion by changing pointers.
d. Store Next pointers of Both A and B.
e. Make B as next of pointer A and next of pointer B is next of pointer A, By doing this we insert a node of ListB in ListA.
f. Move pointers of ListA and ListB.

Algorithm working

C++ Program

#include <bits/stdc++.h>

using namespace std;

struct LLNode
    int data;
    struct LLNode* next;

/* Function to insertAtBeginning a node */
void insertAtBeginning(struct LLNode** head, int dataToBeInserted)
    struct LLNode* curr = new LLNode;
    curr->data = dataToBeInserted;
    curr->next = NULL;    
    if(*head == NULL)
            *head=curr; //if this is first node make this as head of list
            curr->next=*head; //else make the curr (new) node's next point to head and make this new node a the head
        //O(1) constant time
//display linked list
void display(struct LLNode**node)
    struct LLNode *temp= *node;
    if (temp==NULL)
        cout<<"Empty linked list"<<endl;
            cout<<temp->data<<" ->";
            temp=temp->next; //move to next node
        //O(number of nodes)

void Merge(struct LLNode *A, struct LLNode **B)
     struct LLNode *current_inA = A, *current_inB = *B;
     struct LLNode *next_inA, *next_inB;
     while (current_inA != NULL && current_inB != NULL)
         //store next pointers of current pointers
         next_inA = current_inA->next;
         next_inB = current_inB->next;
         //adding node from B to A by changing:
         //next of node in B to next in A
         //move pointers
         current_inB->next = next_inA;  
         current_inA->next = current_inB;  
         current_inA = next_inA;
         current_inB = next_inB;
    //change pointer of head of list B
    //remaining nodes
    *B = current_inB;
// Driver program to test above functions
int main()
     struct LLNode *ListA = NULL, *ListB = NULL;
     insertAtBeginning(&ListA, 8);
     insertAtBeginning(&ListA, 7);
     insertAtBeginning(&ListA, 6);
     insertAtBeginning(&ListB, 5);
     insertAtBeginning(&ListB, 4);
     insertAtBeginning(&ListB, 3);
     insertAtBeginning(&ListB, 2);
     insertAtBeginning(&ListB, 1);

     cout<<"Linked List A: ";
     cout<<"Linked List B: ";
     cout<<"Merging List B into List A at alternate positions in A";
     cout<<"\nOutput Linked List A: ";
     cout<<"Output Linked List B: ";
     return 0;


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