思路

用两个栈st_in和st_out实现队列，st_in负责实现“入队”操作，st_out负责实现“出队”操作；

队列操作

（2）push()：执行st_in.push()；
（3）pop()：如果st_out不为空则执行st_out.pop()；如果为空，则将st_in中的元素全部入栈到st_out中；
（4）peek()：同上类似；
（5）empty()：如果两个栈都为空，则队列为空。

class MyQueue {
public:
    /** Initialize your data structure here. */
    stack<int> st_in;
    stack<int> st_out;

    MyQueue() {

    }

    /** Push element x to the back of queue. */
    void push(int x) {
        st_in.push(x);
    }

    /** Removes the element from in front of queue and returns that element. */
    int pop() {
        if(st_out.empty()){
            while(st_in.size()){
                st_out.push(st_in.top());
                st_in.pop();
            }
        }
        int el = st_out.top();
        st_out.pop();
        return el;
    }

    /** Get the front element. */
    int peek() {
        if(st_out.empty()){
            while(st_in.size()){
                st_out.push(st_in.top());
                st_in.pop();
            }
        }
        return st_out.top();
    }

    /** Returns whether the queue is empty. */
    bool empty() {
        return st_out.empty() && st_in.empty();
    }
};

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