LeetCode 232 Implement Queue using Stacks（利用栈实现队列）

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).

题目大意：利用栈实现队列的push、pop、peek和empty操作。

解题思路：用两个栈s1和s2，入队操作在s1上进行，出队时先检查s2是否为空，如果s2为空，则将s1中的元素压入s2中；如果s2不为空，则直接弹出s2的栈顶元素。

代码如下：

class MyQueue {public:    /** Initialize your data structure here. */    MyQueue() {        while(s1.size() || s2.size()){            s1.pop();            s2.pop();        }    }        /** 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 tmp = MyQueue::peek();        s2.pop();        return tmp;    }        /** Get the front element. */    int peek() {        if(s2.empty()){            while(s1.size()){                s2.push(s1.top());                s1.pop();            }        }        int tmp = s2.top();        return tmp;    }        /** Returns whether the queue is empty. */    bool empty() {        return s1.empty() && s2.empty();    }private:    stack<int> s1;    stack<int> s2;};