题目

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.

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).

分析

用队列实现栈。

用两个队列，其中一个队列用户存储当前元素，另一个辅助队列作为pop和top操作时的临时存储。并利用flag标志，表示存储当前所有元素的队列。

AC代码

class Stack {

public:

    // Push element x onto stack.

    void push(int x) {

        que[flag].push(x);

    }

    // Removes the element on top of the stack.

    void pop() {

        while (que[flag].size() > 1)

        {

            que[1 - flag].push(que[flag].front());

            que[flag].pop();

        }//while

        que[flag].pop();

        flag = 1 - flag;

    }

    // Get the top element.

    int top() {

        while (que[flag].size() > 1)

        {

            que[1 - flag].push(que[flag].front());

            que[flag].pop();

        }//while

        int ret = que[flag].front();

        que[1 - flag].push(que[flag].front());

        que[flag].pop();

        flag = 1 - flag;

        return ret;

    }

    // Return whether the stack is empty.

    bool empty() {

        return que[flag].empty();

    }

private:

    queue<int> que[2];

    int flag = 0; //作为存储队列

};

GitHub测试程序源码