LeetCode 225. 用队列实现栈

思路：
一个队列用于备份，先将原队列最后一位元素之前的元素弹出，push进备份队列，后再还原

class MyStack {
public:
    queue<int> queue1;
    queue<int> queue2; //备份
    MyStack() {

    }
    
    void push(int x) {
        queue1.push(x);
    }
    
    int pop() {
        int res;
        int n = queue1.size() - 1;
        while (n --) {
            queue2.push(queue1.front());
            queue1.pop();
        }

        res = queue1.front();
        queue1.pop();

        while (queue2.size()) {
            queue1.push(queue2.front());
            queue2.pop();
        }

        return res;
    }
    
    int top() {
        int res = pop();
        queue1.push(res);

        return res;
    }
    
    bool empty() {
        return queue1.empty();
    }
};

/**
 * Your MyStack object will be instantiated and called as such:
 * MyStack* obj = new MyStack();
 * obj->push(x);
 * int param_2 = obj->pop();
 * int param_3 = obj->top();
 * bool param_4 = obj->empty();
 */