SinglyLinkedListNode* mergeLists(SinglyLinkedListNode* head1, SinglyLinkedListNode* head2) {

if(head1 ==nullptr) { return head2; }

if(head2 == nullptr) { return head1; }

stack temp; while (head1 != nullptr && head2 != nullptr ) {

if(head1->data <= head2->data)
{

   temp.push(head1->data );
   head1 = head1->next;

}else {
   temp.push(head2->data );
   head2 = head2->next;

}

}

while (head1 != nullptr) { temp.push(head1->data );

        head1 = head1->next;

}

while (head2 != nullptr) { temp.push(head2->data );

head2 = head2->next;

}

SinglyLinkedListNode* res = nullptr;

while(!temp.empty()){

int tp = temp.top();

temp.pop();

SinglyLinkedListNode *temp = new SinglyLinkedListNode(tp); temp->next = res; res = temp; }

return res;

}