Adding code for merging two sorted linked lists.

   /*The linked list node structure is as follows:
    
    template <typename T>
    class Node {
        public:
        T data;
        Node* next;
        Node(T data) {
            next = NULL;
            this->data = data;
        }
        ~Node() {
            if (next != NULL) {
                delete next;
            }
        }
    };
*/
/*The function for merging two sorted linked lists is as:*/
void merge_two_sorted_LL(Node<int>* firstlist, Node<int>* secondlist) {
    
    /*Denoting current node of first linked list by curr1 */
    Node* curr1 = firstlist;     
    Node* next1 = curr1 -> next;
    
    /*Denoting current node of second linked list by curr2 */
    Node* curr2 = secondlist;
    Node* next2 = curr2 -> next;
    
    while(next1 != NULL && curr2 != NULL) {
        
        if( (curr2 -> data >= curr1 -> data ) 
           && ( curr2 -> data <= next1 -> data)) {
            
            curr1 -> next = curr2;
            curr2 -> next = next1;
            curr1 = curr2;
            curr2 = next2;
        }
        else {
            
        }
        
        
    }
    
    
}

Node<int>* returning_final_linkedlist(Node<int>* first, Node<int>* second)
{
    if(firstlist == NULL)
        return secondlist;
    
    if(secondlist == NULL)
        return firstlist;
    
    if(first -> data <= second -> data ){
       merge_two_sorted_LL(firstlist, secondlist);
    }
    else
    {
       merge_two_sorted_LL(secondlist, firstlist);
    }
    
    
}