C++ Program For In-Place Merge Two Linked Lists Without Changing Links Of First List

// C++ Program to merge two sorted linked lists 
// without using any extra space and without 
// changing links of first list
#include <bits/stdc++.h>
using namespace std;

// Structure for a linked list node 
struct Node
{
    int data;
    struct Node *next;
};

/* Given a reference (pointer to pointer) 
   to the head of a list and an int, push 
   a new node on the front of the list. */
void push(struct Node** head_ref, 
          int new_data)
{
    // Allocate node  
    struct Node* new_node =
           (struct Node*) malloc(sizeof(struct Node));

    // Put in the data  
    new_node->data = new_data;

    /* Link the old list off the 
       new node */
    new_node->next = (*head_ref);

    /* Move the head to point to 
       the new node */
    (*head_ref) = new_node;
}

// Function to merge two sorted 
// linked lists LL1 and LL2 without 
// using any extra space.
void mergeLists(struct Node *a, 
                struct Node * &b)
{
    // Run till either one of a or b 
    // runs out
    while (a && b)
    {
        // For each element of LL1,
        // compare it with first element 
        // of LL2.
        if (a->data > b->data)
        {
            // Swap the two elements involved
            // if LL1 has a greater element
            swap(a->data, b->data);

            struct Node *temp = b;

            // To keep LL2 sorted, place 
            // first element of LL2 at its 
            // correct place
            if (b->next && b->data > 
                b->next->data)
            {
                b = b->next;
                struct Node *ptr= b, 
                            *prev = NULL;

                // Find mismatch by traversing 
                // the second linked list once
                while (ptr && ptr->data < 
                       temp->data)
                {
                    prev = ptr;
                    ptr = ptr -> next;
                }

                // correct the pointers
                prev->next = temp;
                temp->next = ptr;
            }
        }

        // Move LL1 pointer to next element
        a = a->next;
    }
}

// Code to print the linked link
void printList(struct Node *head)
{
    while (head)
    {
        cout << head->data << "->" ;
        head = head->next;
    }
    cout << "NULL" << endl;
}

// Driver code
int main()
{
    struct Node *a = NULL;
    push(&a, 10);
    push(&a, 8);
    push(&a, 7);
    push(&a, 4);
    push(&a, 2);

    struct Node *b = NULL;
    push(&b, 12);
    push(&b, 3);
    push(&b, 1);

    mergeLists(a, b);

    cout << "First List: ";
    printList(a);

    cout << "Second List: ";
    printList(b);

    return 0;
}