225. Implement Stack using Queues

class Stack {
public:
  // Push element x onto stack.
  void push(int x) {
    Q.push(x);
  }

  // Removes the element on top of the stack.
  void pop() {
    int len = Q.size();
    for (int i = 1; i < len; i++){
      Q.push(Q.front());
      Q.pop();
    }
    Q.pop();
  }

  // Get the top element.
  int top() {
    int len = Q.size();
    for (int i = 1; i < len; i++){
      Q.push(Q.front());
      Q.pop();
    }
    int front = Q.front();
    Q.push(front);
    Q.pop();
    return front;
  }

  // Return whether the stack is empty.
  bool empty() {
    return Q.empty();
  }
private:
  queue<int> Q;
};