LeetCode（225） Implement Stack using Queues

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