用隊列實現棧。這個實現方法十分的簡單，就是在push這一步的時候直接變成逆序。

class MyStack {
private:
    queue<int> q;
    queue<int> q2;
public:
    /** Initialize your data structure here. */
    MyStack() {
        
    }
    
    /** Push element x onto stack. */
    void push(int x) {
        q.push(x);
        for(int i=0
 
;i<q.size()-1;i++)
        {
            q.push(q.front());
            q.pop();
        }
    }
    
    /** Removes the element on top of the stack and returns that element. */
    int pop() {
        int a = q.front();
        q.pop();
        return a;
    }
    
    /** Get the top element. */
    int
 
 top() {
        return q.front();
    }
    
    /** Returns whether the stack is empty. */
    bool empty() {
        return q.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();
 
 
*/

【easy】225. Implement Stack using Queues