Queue Using Tow Stacks

/*Given will be queries of 2 types 1(enqueue) and 2(dequeue) in queue.
u will be using only 2 stacks to do it(means implement a queue), no other library than stack ops are avaiable.

Approach:
here the ultimate judgement will based on ordering of getting/deleting the oldest element added. but stack's behaviour is just opposite.
So idea is to use two stack and double perform. so during enqueue, no worries as no checks will be there but on getting top() or dequeue() query,
it will expect the oldest element, so second stack can be used to store the first stack's elements in opposite order and when last element is reached, 
That is the required one.

#include <bits/stdc++.h> 
class Queue{
    stack<int> stk1, stk2;
    public:
    bool enqueue(int X){
        stk1.push(X);
        return true;
    }
    int dequeue(){
        if(stk1.empty()) return -1;
        while(stk1.size()>1){
            int topX = stk1.top();
            stk1.pop();
            stk2.push(topX);
        }
        int ans = stk1.top();
        stk1.pop();
        while(stk2.size()){
            int topX = stk2.top();
            stk2.pop();
            stk1.push(topX);
        }
        return ans;
    }
};

//T.C. : O(N)
//S.C. : O(1) other than given DS