[leetcode-225]Implement Stack using Queues(c++)

问题描述：
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).
Update (2015-06-11):
The class name of the Java function had been updated to MyStack instead of Stack.

分析：这里可能最讨厌的一点就是pop的过程，那这里需要两个队列来实现，我是使用一个主队列，一个辅助队列，在pop完之后，将辅助队列的值恢复到 主队列中。
有一些人是使用两个队列，两个队列不分主次。使用一个索引来标示到底使用哪个队列。

此外，我把头节点用一个属性值来标示，这样当取top值时，就直接返回这个属性值即可。

代码如下：0ms

class Stack {
    queue<int> q;
    queue<int> tmp;
    int topValue;
public:
    // Push element x onto stack.
    void push(int x) {
        q.push(x);
        topValue = x;
    }

    // Removes the element on top of the stack.
    void pop() {
        while (q.size() > 1) {
            tmp.push(q.front());
            q.pop();
        }
        q.pop();
        while (tmp.size() > 0) {
            q.push(tmp.front());
            tmp.pop();
        }
        topValue = q.back();
    }

    // Get the top element.
    int top() {
        return topValue;
    }

    // Return whether the stack is empty.
    bool empty() {
        return q.empty();
    }
};