1. 用队列实现栈。

225. 用队列实现栈 - 力扣（LeetCode）

一个队列无法实现栈

尝试使用两个队列 

1)push元素的时候应当放在那里？哪个队列不为空就放在哪里

2）出栈的时候，出不为空的队列size-1元素，剩余元素是要出栈的元素

import java.util.LinkedList;
import java.util.Queue;class MyStack {private Queue<Integer> qu1;private Queue<Integer> qu2;public MyStack() {qu1 = new LinkedList<>();qu2 = new LinkedList<>();}public void push(int x) {//放到不为空的队列if(!qu1.isEmpty()) {qu1.offer(x);}else if(!qu2.isEmpty()) {qu2.offer(x);}else {//如果都是空的 放到第一个qu1.offer(x);}}public int pop() {//两个队列都是空的： 栈为空if(empty()) {return -1;}if(!qu1.isEmpty()) {int currentSize = qu1.size();for (int i = 0; i < currentSize-1; i++) {int x = qu1.poll();qu2.offer(x);}return qu1.poll();//最后一个数据返回}if(!qu2.isEmpty()) {int currentSize = qu2.size();for (int i = 0; i < currentSize-1; i++) {int x = qu2.poll();qu1.offer(x);}return qu2.poll();//最后一个数据返回}return -1;}//peek方法public int top() {if(empty()) {return -1;}if(!qu1.isEmpty()) {int currentSize = qu1.size();int x = -1;for (int i = 0; i < currentSize; i++) {x = qu1.poll();qu2.offer(x);}return x;//最后一个数据返回}if(!qu2.isEmpty()) {int currentSize = qu2.size();int x = -1;for (int i = 0; i < currentSize; i++) {x = qu2.poll();qu1.offer(x);}return x;//最后一个数据返回}return -1;}public boolean empty() {return qu1.isEmpty() && qu2.isEmpty();}
}

2.用栈实现队列。

. - 力扣（LeetCode）

class MyQueue {//定义两个栈private Stack<Integer> s1;private Stack<Integer> s2;//入的时候统一入到s1中//出的时候统一出到s2中//实例化两个栈public MyQueue() {s1=new Stack<>();s2=new Stack<>();}public void push(int x) {s1.push(x);}public int pop() {if(empty()){return -1;}if(s2.empty()){while(!s1.empty()){s2.push(s1.pop());}}return s2.pop();}public int peek() {if(s1.empty()){return -1;}if(s2.empty()){while(!s1.empty()){s2.push(s1.pop());}}return s2.peek();}public boolean empty() {return s1.empty()&&s2.empty();}
}