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；
  ?

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  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();  */