【LeetCode 232: Implement Queue using Stacks】

Description:

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).

// 代码实现class Queue {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) {        if(!s2.empty()){            s2.pop();        }        else{            while(!s1.empty()){                s2.push(s1.top());                s1.pop();            }            s2.pop();        }    }    // Get the front element.    int peek(void) {        int nRet = 0;        if(!s2.empty()){            nRet = s2.top();        }        else{            while(!s1.empty()){                s2.push(s1.top());                s1.pop();            }            nRet = s2.top();        }        return nRet;    }    // Return whether the queue is empty.    bool empty(void) {        bool bRet = false;        bRet = s1.empty() ;        bRet = bRet && s2.empty();        return bRet;    }private:    stack<int> s1, s2; // ；两个栈};

小结与思考：

注意各种数据结构的特性；思考使用两个队列实现一个栈；