用两个栈实现队列

请用栈实现一个队列，支持如下四种操作：

• push(x) – 将元素x插到队尾；
• pop() – 将队首的元素弹出，并返回该元素；
• peek() – 返回队首元素；
• empty() – 返回队列是否为空；

  class MyQueue {
  public:
      /** Initialize your data structure here. */
      stack<int> a, b;
      
      MyQueue() {
          
      }
      
      /** Push element x to the back of queue. */
      void push(int x) {
          a.push(x);
      }
      
      /** Removes the element from in front of queue and returns that element. */
      int pop() {
          while (a.size() > 1) {
              b.push(a.top());
              a.pop();
          }
          int t = a.top(); a.pop();
          while (b.size()) {
              a.push(b.top());
              b.pop();
          }
          return t;
      }
      
      /** Get the front element. */
      int peek() {
          while (a.size() > 1) {
              b.push(a.top());
              a.pop();
          }
          int t = a.top();
          while (b.size()) {
              a.push(b.top());
              b.pop();
          }
          return t;
      }
      
      /** Returns whether the queue is empty. */
      bool empty() {
          return a.empty();
      }
  };
  
  /**
   * Your MyQueue object will be instantiated and called as such:
   * MyQueue obj = MyQueue();
   * obj.push(x);
   * int param_2 = obj.pop();
   * int param_3 = obj.peek();
   * bool param_4 = obj.empty();
   */