232 用栈实现队列

思路：

• 准备两个栈，一个push栈，一个pop栈

• 执行push操作时，只向push栈中添加元素

• 执行pop操作时，若pop栈为空，则先将push栈中的元素全倒入到pop栈中，然后弹出pop栈的栈顶

• 执行peek操作时，和pop操作基本相同

• 当push栈和pop栈均为空时，队列为空

• *代码：**

  class MyQueue {
      private Stack<Integer> stackPop;
      private Stack<Integer> stackPush; 
      /** Initialize your data structure here. */
      public MyQueue() {
          this.stackPop = new Stack<>();
          this.stackPush = new Stack<>();
      }
      
      /** Push element x to the back of queue. */
      public void push(int x) {
          stackPush.push(x);
      }
      
      /** Removes the element from in front of queue and returns that element. */
      public int pop() {
          //当pop栈为空时，将push栈中的元素都加入到pop栈中来
          if(stackPop.empty()){
              while(!stackPush.empty())
                  stackPop.push(stackPush.pop());
          }
          //pop不空时，直接弹出pop栈的栈顶
          return stackPop.pop();
      }
      
      /** Get the front element. */
      //除了最后一行外，和pop操作基本一致
      public int peek() {
          //当pop栈为空时，将push栈中的元素都加入到pop栈中来
          if(stackPop.empty()){
              while(!stackPush.empty())
                  stackPop.push(stackPush.pop());
          }
          return stackPop.peek();
      }
      
      /** Returns whether the queue is empty. */
      public boolean empty() {
          return stackPush.empty() && stackPop.empty();
      }
  }