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

        使用堆栈实现队列。

        使用两个堆栈，push的时候正常放入堆栈1，pop的时候，如果堆栈2为空，就把堆栈1的数据全部放入堆栈2（此时数据就倒序了），这时候从堆栈2的顶部就能取到最早进入的数据。

class MyQueue {
public:
    stack<int> s1;
    stack<int> s2;
    /** Initialize your data structure here. */
    MyQueue() {
        
    }
    
    /** Push element x to the back of queue. */
    void push(int x) {
        s1.push(x);
    }
    
    /** Removes the element from in front of queue and returns that element. */
    int pop() {
        int res = peek();
        s2.pop();
        return res;
    }
    
    /** Get the front element. */
    int peek() {
        if(s2.empty())
        {
            while(!s1.empty())
            {
                s2.push(s1.top());
                s1.pop();
            }
        }
        return s2.top();        
    }
    
    /** Returns whether the queue is empty. */
    bool empty() {
        return s1.empty() && s2.empty();
    }
};

/**
 * 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();
 */