一、链式队列

1. 使用双向链表实现队列，可以采用尾入，头出 也可以采用 头入、尾出 (LinkedList采用尾入、头出)

2. 下面代码实现的是尾入、头出：

package bageight;/*** Created with IntelliJ IDEA.* Description:* User: tangyuxiu* Date: 2024-08-22* Time: 16:44*/
public class MyQueue<E> {/* 使用内部类实现链表的节点 */private class ListNode<E> {E val;ListNode<E> prev;ListNode<E> next;public ListNode(E val) {this.val = val;}}private ListNode<E> head;private ListNode<E> last;/* push尾入 */public void offer(E data) {ListNode<E> newNode = new ListNode<>(data);//1. 如果链表为nullif (this.head == null) {this.head = this.last = newNode;return;}//2. 尾插this.last.next = newNode;newNode.prev = this.last;this.last = newNode;}/* poll头出 */public E poll() {//1. 如果链表为nullif (this.head == null) {return null;}//2. 头出E del = this.head.val;if (this.head.next == null) {//链表只有一个节点this.head = this.last = null;} else {//链表有多个节点this.head = this.head.next;this.head.prev = null;}return del;}/* peek */public E peek() {//1. 如果链表为nullif (this.head == null) {return null;}//2. 正常return this.head.val;}/* size */public int size() {int count = 0;ListNode<E> cur = this.head;while (cur != null) {count++;cur = cur.next;}return count;}
}

二、循环队列

622. 设计循环队列 - 力扣（LeetCode）

【难点】：

① 到底怎么判断什么时候队列为null，什么时候队列满了

② fast++，如何不越界：fast = fast % len 或 第三种做法要写成fast = (fast + 1) % len

【等待补充】使用flag的方式如何编写代码。

2.1 使用uesdSize判断空满

class MyCircularQueue {private int[] elem;//由于不是泛型类，且依据题意放入数组中的数据都是int类型的，//因此如此定义private int head;//队头位置private int last;//待插入数据位置private int uesdSize;//队列有效数据个数public MyCircularQueue(int k) {this.elem = new int[k];}//尾插public boolean enQueue(int value) {if (this.isFull()) {return false;}this.elem[this.last] = value;this.last++;//使得last的下标一直维持在0~elem.length-1this.last = this.last % this.elem.length;this.uesdSize++;return true;}//头删public boolean deQueue() {if (this.isEmpty()) {return false;}//头删是让head的值++，而不是--🙀this.head++;this.head = this.head % this.elem.length;this.uesdSize--;return true;}//获取队头元素的值public int Front() {if (this.isEmpty()) {return -1;}return this.elem[this.head];}//获取队尾元素的值public int Rear() {if (this.isEmpty()) {return -1;}if (this.last == 0) {return this.elem[this.elem.length - 1];} return this.elem[this.last - 1];}public boolean isEmpty() {return this.uesdSize == 0;}public boolean isFull() {return this.uesdSize == this.elem.length;}
}

2.2 浪费一个数组空间来判断空满

class MyCircularQueue {private int[] elem;//由于不是泛型类，且依据题意放入数组中的数据都是int类型的，//因此如此定义private int head;//队头位置private int last;//待插入数据位置public MyCircularQueue(int k) {this.elem = new int[k + 1];}//尾插public boolean enQueue(int value) {if (this.isFull()) {return false;}this.elem[this.last] = value;this.last++;//使得last的下标一直维持在0~elem.length-1this.last = this.last % this.elem.length;return true;}//头删public boolean deQueue() {if (this.isEmpty()) {return false;}//头删是让head的值++，而不是--🙀this.head++;this.head = this.head % this.elem.length;return true;    }//获取队头元素的值public int Front() {if (this.isEmpty()) {return -1;}return this.elem[this.head];}//获取队尾元素的值public int Rear() {if (this.isEmpty()) {return -1;}if (this.last == 0) {return this.elem[this.elem.length - 1];} return this.elem[this.last - 1];}public boolean isEmpty() {return this.head == this.last;}public boolean isFull() {return (this.last + 1) % this.elem.length == this.head;//🙀，想一想特殊情况就知道为什么这么写了}
}