mergeSortedlinkedlist.cpp

void solve(Node<int>* first, Node<int>* second){

    //if only one element is present in the list
    if(first -> next == NULL){
        first -> next = second;
        return first;
    }
      Node<int>* curr1 = first;
      Node<int>* next1 = curr1 -> next;
      Node<int>* curr2 = second;
      Node<int>* next2 = curr2 -> next;

      while(next !=NULL && curr2 != NULL){
        if((curr2 -> data >= curr1 -> data) && 
        (curr2 -> data <= next1 -> data)){
            curr1 -> next = curr2;
            next2 = curr2 -> next;
            curr2 -> next = next1;
        //update pointers
            curr1 = curr2;
            curr2 = next2;
        }
        else{
            //curr1 and next1 ko aage badhana padega
            curr1 = next1;
            next1 = next1 -> next;
            if(next1 == NULL){
                curr1 -> next = curr2;
                return first;
            }
        }
      }
      return first;
}

Node<int>* sortTwoLists(Node<int>* first, Node<int>* second)
{
    
    
   if(first == NULL)
   return second;

   if(second == NULL)
   return first;

   if(first -> data <= second -> data){
     return solve(first, second);
   } 
   else{
    return solve(second, first);
   }
   
}