leetcode 225. Implement Stack using Queues

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).

用队列实现栈。每次push的时候，进行反转或每次top的时候进行反转。

class MyStack {
public:
    queue<int> q1;
    queue<int> q2;
    /** Initialize your data structure here. */
    MyStack() {
        while(!q1.empty()) q1.pop();
        while(!q2.empty()) q2.pop(); 
    }
    
    /** Push element x onto stack. */
    void push(int x) {
        while(!q1.empty()) {
            int x = q1.front();q1.pop();
            q2.push(x);
        }
        q1.push(x);
        while(!q2.empty()) {
            int x = q2.front();q2.pop();
            q1.push(x);
        }
    }
    
    /** Removes the element on top of the stack and returns that element. */
    int pop() {
        int x = q1.front();q1.pop();
        return x;
    }
    
    /** Get the top element. */
    int top() {
        return q1.front();
    }
    
    /** Returns whether the stack is empty. */
    bool empty() {
        return q1.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();
 */