1、用栈实现队列的功能

思路 ：

• 队列的特点 FIFO
• 栈的特点 FILO

用两个栈，一个做为入队栈，一个作为出队栈，每次出队，都从出队栈出，如果出队栈无数据，从入队栈出栈，再入出队栈

package com.lxd.leetcode.demo.stack;import java.util.Arrays;
import java.util.Stack;/*** @ClassName StackImplQueue* @Description: 用栈实现队列功能* @Author xiaod* @Date 2020/7/3* @Version V1.0**/
public class StackImplQueue {/****  - 队列的特点 FIFO*  - 栈的特点 FILO** 用两个栈，一个做为入队栈，一个作为出队栈，每次出队，都从出队栈出，如果出队栈无数据，从入队栈出栈，再入出队栈*/private static class QueueByStack<T>{private Stack<T> inStack;private Stack<T> outStack;public  QueueByStack(Stack inStack, Stack outStack){this.inStack = inStack;this.outStack = outStack;}/*** 入队* @param t*/public void push(T t){inStack.push(t);}/**** 获取队首元素并移除* @return*/public T pop(){if(!outStack.isEmpty()){//出队栈有数据return outStack.pop();}else {//出队栈无数据，将入队栈数据出栈压入出队栈while (!inStack.isEmpty()){outStack.push(inStack.pop());}return outStack.pop();}}/**** 获取队首元素* @return*/public T peek(){if(!outStack.isEmpty()){//出队栈有数据return outStack.peek();}else {//出队栈无数据，将入队栈数据出栈压入出队栈while (!inStack.isEmpty()){outStack.push(inStack.pop());}return outStack.peek();}}}public static void main(String[] args) {Stack instack =new Stack();Stack outstack =new Stack();QueueByStack<Integer> queueByStack=  new QueueByStack(instack,outstack);queueByStack.push(44);queueByStack.push(55);queueByStack.push(66);queueByStack.push(77);queueByStack.push(88);queueByStack.push(99);System.out.println( "queueByStack.peek()1 = " + queueByStack.peek());System.out.println( "queueByStack.pop() = " + queueByStack.pop());System.out.println( "queueByStack.peek()2 = " + queueByStack.peek());}
}

2、用队列实现栈的功能

思路：

准备两个队列，一个用作入栈出栈、一个用来在数据出栈时，临时存储出栈数据队列前面的数据，该数据出栈之后，再将队列数据依次入队。

/*** @ClassName QueueImplStack* @Description: 一用队列实现栈的功能* @Author xiaod* @Date 2020/7/3* @Version V1.0**/
public class QueueImplStack {/**** 准备两个队列，一个用作入栈出栈、一个用来在数据出栈时，临时存储出栈数据队列前面的数据，该数据出栈之后，再将队列数据依次入队。*/private static class StackByQueue<T>{/*** 操作出栈入栈的队列*/private Queue<T> operationQueue;/*** 临时存储数据的队列*/private Queue<T> tempQueue;public StackByQueue(Queue operationQueue, Queue tempQueue) {this.operationQueue = operationQueue;this.tempQueue = tempQueue;}/*** 入栈操作* @param t*/public void push(T t){operationQueue.add(t);}/*** 去除栈顶元素并移除* @return*/public T pop(){while (operationQueue.size()>1){tempQueue.add(operationQueue.poll());}T result = operationQueue.poll();while (!tempQueue.isEmpty()){operationQueue.add(tempQueue.poll());}return result;}/*** 查看栈顶元素* @return*/public T peek(){while (operationQueue.size()>1){tempQueue.add(operationQueue.poll());}T result = operationQueue.poll();while (!tempQueue.isEmpty()){operationQueue.add(tempQueue.poll());}operationQueue.add(result);return result;}}public static void main(String[] args) {Queue operationQueue =new LinkedList();Queue tempQueue =new LinkedList();StackByQueue<Integer> stackByQueue =new StackByQueue<>(operationQueue,tempQueue);stackByQueue.push(44);stackByQueue.push(55);stackByQueue.push(66);stackByQueue.push(77);stackByQueue.push(88);System.out.println( "stackByQueue.peek()1 = " + stackByQueue.peek());System.out.println( "stackByQueue.pop() = " + stackByQueue.pop());System.out.println( "stackByQueue.peek()2 = " + stackByQueue.peek());}
}