1. ArrayList的缺陷

由于其底层是一段连续空间，当在ArrayList任意位置插入或者删除元素时，就需要将后序元素整体往前或者往后搬移，时间复杂度为O(n)，效率比较低，因此ArrayList不适合做任意位置插入和删除比较多的场景。因此：java集合中又引入了LinkedList，即链表结构。

2. 链表

2.1 链表的概念及结构

链表是一种物理存储结构上非连续存储结构，数据元素的逻辑顺序是通过链表中的引用链接次序实现的 。

实际中链表的结构非常多样，以下情况组合起来就有8种链表结构：

1. 单向或者双向

2. 带头或者不带头

3. 循环或者非循环

虽然有这么多的链表的结构，但是我们重点掌握两种:

• 无头单向非循环链表：结构简单，一般不会单独用来存数据。实际中更多是作为其他数据结构的子结构，如哈希桶、图的邻接表等等。另外这种结构在笔试面试中出现很多。

• 无头双向链表：在Java的集合框架库中LinkedList底层实现就是无头双向循环链表。

2.2 链表的实现

public class SingleLinkedList {static class ListNode {public ListNode next;public int val;public ListNode(int val) {this.val = val;}}public ListNode head;public void createList() {ListNode node1 = new ListNode(12);ListNode node2 = new ListNode(23);ListNode node3 = new ListNode(34);ListNode node4 = new ListNode(45);ListNode node5 = new ListNode(56);node1.next = node2;node2.next = node3;node3.next = node4;node4.next = node5;this.head = node1;}public void addFirst(int data){ListNode listNode = new ListNode(data);listNode.next = head;head = listNode;}//尾插法public void addLast(int data){ListNode listNode = new ListNode(data);ListNode cur = head;if (cur == null) {head = listNode;return;}while (cur.next != null) {cur = cur.next;}cur.next = listNode;}//任意位置插入,第一个数据节点为0号下标public void addIndex(int index,int data){ListNode listNode = new ListNode(data);if (index < 0 || index > size()) {System.out.println("index位置不合法");return;}if (index == 0) {addFirst(data);return;}if (index == size()) {addLast(data);return;}ListNode cur = findIndexSubOne(index);listNode.next = cur.next;cur.next = listNode;}private ListNode findIndexSubOne(int index) {ListNode cur = head;for (int i = 0; i < index-1; i++) {cur = cur.next;}return cur;}//查找是否包含关键字key是否在单链表当中public boolean contains(int key){ListNode cur = head;while (cur != null) {if (cur.val == key) {return true;}cur = cur.next;}return false;}//删除第一次出现关键字为key的节点public void remove(int key){ListNode cur = head;if (cur.val == key) {head = cur.next;return;}while (cur.next != null) {if (cur.next.val == key) {cur.next = cur.next.next;return;}cur = cur.next;}}//删除所有值为key的节点public void removeAllKey(int key){if (head == null) {return;}ListNode prev = head;ListNode cur = head.next;while (cur != null) {if (cur.val == key) {prev.next = cur.next;} else {prev = cur;}cur = cur.next;}if (head.val == key) {head = head.next;}}//得到单链表的长度public int size(){int sz = 0;ListNode cur = head;while (cur != null) {sz++;cur = cur.next;}return sz;}public void clear() {ListNode cur = head;while (cur != null) {ListNode curN = cur.next;cur.next = null;cur = curN;}head = null;}public void display() {ListNode cur = head;while (cur != null) {System.out.print(cur.val + " ");cur = cur.next;}System.out.println();}
}

public class Test {public static void main(String[] args) {SingleLinkedList singleLinkedList = new SingleLinkedList();singleLinkedList.createList();singleLinkedList.display();singleLinkedList.addFirst(10);singleLinkedList.display();singleLinkedList.addLast(67);singleLinkedList.display();System.out.println(singleLinkedList.size());singleLinkedList.addIndex(0, 9);singleLinkedList.display();singleLinkedList.addIndex(8, 78);singleLinkedList.display();singleLinkedList.addIndex(2, 11);singleLinkedList.display();singleLinkedList.addIndex(-1, 11);System.out.println(singleLinkedList.contains(78));System.out.println(singleLinkedList.contains(9));System.out.println(singleLinkedList.contains(34));System.out.println(singleLinkedList.contains(99));singleLinkedList.remove(23);singleLinkedList.display();singleLinkedList.remove(78);singleLinkedList.display();singleLinkedList.remove(9);singleLinkedList.display();singleLinkedList.addLast(10);singleLinkedList.addLast(10);singleLinkedList.addLast(10);singleLinkedList.addLast(10);singleLinkedList.display();singleLinkedList.removeAllKey(10);singleLinkedList.display();singleLinkedList.clear();singleLinkedList.display();}
}

执行结果

12 23 34 45 56 
10 12 23 34 45 56 
10 12 23 34 45 56 67 
7
9 10 12 23 34 45 56 67 
9 10 12 23 34 45 56 67 78 
9 10 11 12 23 34 45 56 67 78 
index位置不合法
true
true
true
false
9 10 11 12 34 45 56 67 78 
9 10 11 12 34 45 56 67 
10 11 12 34 45 56 67 
10 11 12 34 45 56 67 10 10 10 10 
11 12 34 45 56 67