232. Implement Queue using Stacks

Implement the following operations of a queue using stacks.

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

用st1保存入队的数。
用st2来删除出队的数,此时为保证入队顺序来显示队头，需要逆序，此时st1就为空。
用st2.empty做标记，来记录是否进行过逆序操作。

class Queue {public:    // Push element x to the back of queue.    void push(int x)     {        while(!st2.empty())        {            st1.push(st2.top());            st2.pop();        }        st1.push(x);    }    // Removes the element from in front of queue.    void pop(void) {        while(!st1.empty())        {            st2.push(st1.top());            st1.pop();        }        st2.pop();    }    // Get the front element.    int peek(void)    {        while(!st1.empty())        {            st2.push(st1.top());            st1.pop();        }        return st2.top();    }    // Return whether the queue is empty.    bool empty(void)    {        return st1.empty() && st2.empty();        }private:    stack<int> st1;    stack<int> st2;};