## 题目描述

Implement the following operations of a stack using queues.

• `​push(x)​`​ – Push element x onto stack.
• `​pop()​`​ – Removes the element on top of the stack.
• `​top()​`​ – Get the top element.
• `​empty()​`​​ – Return whether the stack is empty.
Example:
`MyStack stack = new MyStack();stack.push(1);stack.push(2);  stack.top();   // returns 2stack.pop();   // returns 2stack.empty(); // returns false`

Notes:

• You must use only standard operations of a queue – which means only push to back, peek/pop from front, size, and is empty operations are valid.
• Depending on your language, queue may not be supported natively. You may simulate a queue by using a list or deque (double-ended queue), as long as you use only standard operations of a queue.
• You may assume that all operations are valid (for example, no pop or top operations will be called on an empty stack).

## AC 代码

`class MyStack {public:    /** Initialize your data structure here. */    MyStack() {            }        /** Push element x onto stack. */    void push(int x) {        queue<int> que;                while(!m_stk.empty())        {            que.push(m_stk.front());            m_stk.pop();        }                m_stk.push(x);                while(!que.empty())        {            m_stk.push(que.front());            que.pop();        }    }        /** Removes the element on top of the stack and returns that element. */    int pop() {        int t = top();        m_stk.pop();                return t;    }        /** Get the top element. */    int top() {        return m_stk.front();    }        /** Returns whether the stack is empty. */    bool empty() {        return m_stk.empty();    }    private:    queue<int> m_stk;};/** * Your MyStack object will be instantiated and called as such: * MyStack obj = new MyStack(); * obj.push(x); * int param_2 = obj.pop(); * int param_3 = obj.top(); * bool param_4 = obj.empty(); */`