<?php    /**  232. 用栈实现队列  使用栈实现队列的下列操作：    push(x) -- 将一个元素放入队列的尾部。  pop() -- 从队列首部移除元素。  peek() -- 返回队列首部的元素。  empty() -- 返回队列是否为空。  示例:    MyQueue queue = new MyQueue();    queue.push(1);  queue.push(2);  queue.peek();  // 返回 1  queue.pop();   // 返回 1  queue.empty(); // 返回 false   */  class MyQueue {      /**       * Initialize your data structure here.       */      private $stackPush;      private $stackPop;        function __construct() {          $this->stackPush = new SplStack();          $this->stackPop = new SplStack();      }        /**       * Push element x to the back of queue.       * @param Integer $x       * @return NULL       */      function push($x) {          $this->stackPush->push($x);      }        /**       * Removes the element from in front of queue and returns that element.       * @return Integer       */      function pop() {          if ($this->stackPop->isEmpty()){              $this->shift();          }          return $this->stackPop->pop();        }        function shift(){          while(!$this->stackPush->isEmpty()){              $this->stackPop->push($this->stackPush->pop());          }      }        /**       * Get the front element.       * @return Integer       */      function peek() {          if ($this->stackPop->isEmpty()){              $this->shift();          }          return $this->stackPop->top();      }        /**       * Returns whether the queue is empty.       * @return Boolean       */      function empty() {          return $this->stackPop->isEmpty() && $this->stackPush->isEmpty();      }  }      /**   * Your MyQueue object will be instantiated and called as such:   * $obj = MyQueue();   * $obj->push($x);   * $ret_2 = $obj->pop();   * $ret_3 = $obj->peek();   * $ret_4 = $obj->empty();   */  $query = new MyQueue();  $query->push(1);  $query->push(2);  echo $query->peek();  // 返回 1  echo $query->pop();   // 返回 1  echo $query->empty(); // 返回 false