LeetCode OJ：Implement Stack using Queues（队列实现栈）

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)

类似上一题的两个栈实现队列，但是有点不同，具体的见下面的注释：

class Stack {
public:
    // Push element x onto stack.
    void push(int x) {
        q1.push(x);
    }

    // Removes the element on top of the stack.
    void pop() {
        int tmp = q1.front();//注意这里的实现和双栈实现队列的方式是不相同的
        q1.pop();
        while(!q1.empty()){
            q2.push(tmp);
            tmp = q1.front();
            q1.pop();
        }
        swap(q1,q2);
    }

    // Get the top element.
    int top() {
        int tmp = q1.front();
        q1.pop();
        while(!q1.empty()){
            q2.push(tmp);
            tmp = q1.front();
            q1.pop();
        }
        q2.push(tmp);
        swap(q1, q2);//用swap即可，无须再向上次那样再倒回去
        return tmp;
    }

    // Return whether the stack is empty.
    bool empty() {
        return q1.empty();            
    }
private:
    queue<int> q1;
    queue<int> q2;
};