https://leetcode.com/problems/implement-queue-using-stacks/submissions/

class MyQueue {
    stact<int> enqueStack;
    stact<int> dequeStack;
public:
    /** Initialize your data structure here. */
    MyQueue() {
        
    }
    
    /** Push element x to the back of queue. */
    void push(int x) {
        enqueStack.push(x);
    }
    
    /** Removes the element from in front of queue and returns that element. */
    int pop() {
        if(dequeStack.empty() && enqueStack.empty()) return NULL;
        if(dequeStack.empty()){
            while(!enqueStack.empty())
            {
                dequeStack.push(enqueStack.top());
                enqueStack.pop();
            }
            int top = dequeStack.top();
            dequeStack.pop();
            return top;
        }
        else {
            int top = dequeStack.top();
            dequeStack.pop();
            return top;
        }
    }
    
    /** Get the front element. */
    int peek() {
        if(dequeStack.empty() && enqueStack.empty()) return NULL;
        if(!dequeStack.empty()){
            return dequeStack.top();
        } else{
            while(!enqueStack.empty()){
                dequeStack.push(enqueStack.top());
                enqueStack.pop();
            }
            return dequeStack.top();
        }
    }
    
    /** Returns whether the queue is empty. */
    bool empty() {
        if(dequeStack.empty() && enqueStack.empty()) return true;
        return false;
    }
};

/**
 * Your MyQueue object will be instantiated and called as such:
 * MyQueue* obj = new MyQueue();
 * obj->push(x);
 * int param_2 = obj->pop();
 * int param_3 = obj->peek();
 * bool param_4 = obj->empty();

Runtime: 0 ms, faster than 100.00% of C++ online submissions for Implement Queue using Stacks.

Memory Usage: 7.1 MB, less than 11.12% of C++ online submissions for Implement Queue using Stacks.