leetccode 225. 用队列实现栈（两个队列模拟）

class MyStack {
public:
    /** Initialize your data structure here. */
    queue<int>a;
    queue<int>b;
    MyStack() {

    }
    
    /** Push element x onto stack. */
    void push(int x) {
        b.push(x);
        while(!a.empty()){
            int y=a.front();
            b.push(y);
            a.pop();
        }
        while(!b.empty()){
            int y=b.front();
            a.push(y);
            b.pop();
        }
    }
    
    /** Removes the element on top of the stack and returns that element. */
    int pop() {
        int y=a.front();
        a.pop();
        return y;
    }
    
    /** Get the top element. */
    int top() {
        return a.front();
    }
    
    /** Returns whether the stack is empty. */
    bool empty() {
        return a.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();
 */