Problem: 225. 用队列实现栈

题目描述：

思路

1.对一个queue模拟栈的操作，同时用一个int类型的变量topElem记录每次每次队列队尾的元素（也即是模拟stack中的stack的栈顶元素）；
2.void push(int x):直接将元素入队，同时用topElem记录该元素；
3.int pop():先将队尾倒数两个元素前的元素出队再入队，再将topElem更新记录尾当前队列的队头元素，再将当前的对头元素出队、入队，最后将当前的对头元素出队返回
4.int top():直接返回topElem即可
5.boolean empty():直接判断队列是否位空即可

Code

class MyStack {Queue<Integer> queue;int topElem;public MyStack() {queue = new LinkedList<>();topElem = 0;}/*** Push the element to the queue** @param x The element to be pushed*/public void push(int x) {queue.offer(x);topElem = x;}/*** Remove the top element of stack** @return int*/public int pop() {int size = queue.size();//remain tow tail element of queuewhile (size > 2) {queue.offer(queue.poll());size--;}//recode the new tail elementtopElem = queue.peek();queue.offer(queue.poll());return queue.poll();}/*** Return the tail element of queue** @return int*/public int top() {return topElem;}/*** Check the empty** @return boolean*/public boolean empty() {return queue.isEmpty();}
}/*** Your MyStack object will be instantiated and called as such:* MyStack obj = new MyStack();* obj.push(x);* int param_2 = obj.pop();* int param_3 = obj.top();* boolean param_4 = obj.empty();*/