Implement Stack using Queues（C++用隊列實現棧）

解題思路：

（1）類比循環鏈表，對於每一剛進入隊列的元素x，使用隊列的pop、push操作，使新進入的元素x移動到隊列的首部

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