Implement the following operations of a queue using stacks.

push(x) – Push element x to the back of queue.

pop() – Removes the element from in front of queue. peek() – Get the front element. empty() – Return whether the queue is empty. Notes: You must use only standard operations of a stack – which means only push to top, peek/pop from top, size, and is empty operations are valid. Depending on your language, stack may not be supported natively. You may simulate a stack by using a list or deque (double-ended queue), as long as you use only standard operations of a stack. You may assume that all operations are valid (for example, no pop or peek operations will be called on an empty queue).

跟前面用队列实现堆原理是差不多的。

class MyQueue {        Stack
     
       s1 = new Stack
      
       ();        Stack
       
         s2 = new Stack
        
         ();        // Push element x to the back of queue.        public void push(int x) {            s1.push(x);        }        // Removes the element from in front of queue.        public void pop() {            while (s1.size() > 1)                s2.push(s1.pop());            s1.pop();            while (s2.size() > 0)                s1.push(s2.pop());        }        // Get the front element.        public int peek() {            while (s1.size() > 0)                s2.push(s1.pop());            int n = s2.peek();            while (s2.size() > 0)                s1.push(s2.pop());            return n;        }        // Return whether the queue is empty.        public boolean empty() {            return s1.isEmpty();        }    }