【Leetcode】Implement Stack using Queues

题目链接：https://leetcode.com/problems/implement-stack-using-queues/
题目：

Implement the following operations of a stack using queues.

• push(x) -- Push element x onto stack.
• pop() -- Removes the element on top of the stack.
• top() -- Get the top element.
• empty() -- Return whether the stack is empty.

Notes:

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

思路：

用两个队列模拟一个栈。每当push、pop的时候都要更新两个队列。

算法：

 class MyStack {
     Queue<Integer> q1 = new LinkedList<Integer>();
     Queue<Integer> q2 = new LinkedList<Integer>();

     // Push element x onto stack.
     public void push(int x) {
         Queue<Integer> tmp = new LinkedList<Integer>();
         q1.offer(x);
         //更新q2队列，将x放到q2正确的位置
         while(!q2.isEmpty()){
             tmp.offer(q2.poll());
         }
         q2.offer(x);
         while(!tmp.isEmpty()){
             q2.offer(tmp.poll());
         }
     }

     // Removes the element on top of the stack.
     public void pop() {
         Queue<Integer> tmp = new LinkedList<Integer>();
         q2.poll();
         int i = 0;
         while(!q1.isEmpty()){
             i = q1.poll();
             if(!q1.isEmpty()){ //q1倒数第一个元素不要了
                 tmp.offer(i);
             }
         }
         while(!tmp.isEmpty()){
             q1.offer(tmp.poll());
         }
     }

     // Get the top element.
     public int top() {
         return q2.peek();
     }

     // Return whether the stack is empty.
     public boolean empty() {
         return q1.isEmpty();
     }
 }