leetcode-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).

用栈实现队列的操作。

很简单，用两个栈即可取到任意一端的元素。

class Queue {
    stack<int> s1;
    stack<int> s2;
public:
    // Push element x to the back of queue.
    void push(int x) {
        s1.push(x);
    }

    // Removes the element from in front of queue.
    void pop(void) {
        while(!s1.empty())
        {
            s2.push(s1.top());
            s1.pop();
        }
        s2.pop();
        while(!s2.empty())
        {
            s1.push(s2.top());
            s2.pop();
        }

    }

    // Get the front element.
    int peek(void) {
        while(!s1.empty())
        {
            s2.push(s1.top());
            s1.pop();
        }
        int p = s2.top();
        while(!s2.empty())
        {
            s1.push(s2.top());
            s2.pop();
        }
        return p;
    }

    // Return whether the queue is empty.
    bool empty(void) {
        return s1.empty();

    }
};