你可以选择使用单链表或者双链表，设计并实现自己的链表。力扣题目
单链表中的节点应该具备两个属性：val 和 next 。val 是当前节点的值，next 是指向下一个节点的指针/引用。
如果是双向链表，则还需要属性 prev 以指示链表中的上一个节点。假设链表中的所有节点下标从 0 开始。
实现 MyLinkedList 类：
MyLinkedList()初始化MyLinkedList对象。int get(int index)获取链表中下标为index的节点的值。如果下标无效，则返回-1。void addAtHead(int val)将一个值为val的节点插入到链表中第一个元素之前。在插入完成后，新节点会成为链表的第一个节点。void addAtTail(int val)将一个值为val的节点追加到链表中作为链表的最后一个元素。void addAtIndex(int index, int val)将一个值为val的节点插入到链表中下标为index的节点之前。如果index等于链表的长度，那么该节点会被追加到链表的末尾。如果index比长度更大，该节点将 不会插入 到链表中。void deleteAtIndex(int index)如果下标有效，则删除链表中下标为index的节点。
示例：
输入
["MyLinkedList", "addAtHead", "addAtTail", "addAtIndex", "get", "deleteAtIndex", "get"]
[[], [1], [3], [1, 2], [1], [1], [1]]
输出
[null, null, null, null, 2, null, 3]
解释
MyLinkedList myLinkedList = new MyLinkedList();
myLinkedList.addAtHead(1);
myLinkedList.addAtTail(3);
myLinkedList.addAtIndex(1, 2); // 链表变为 1->2->3
myLinkedList.get(1); // 返回 2
myLinkedList.deleteAtIndex(1); // 现在，链表变为 1->3
myLinkedList.get(1); // 返回 3
提示：
0 <= index, val <= 1000- 请不要使用内置的 LinkedList 库。
- 调用
get、addAtHead、addAtTail、addAtIndex和deleteAtIndex的次数不超过2000。
解题思路： 1、单链表 2、双链表
单链表
class MyLinkedList {
int length;
ListNode head,tail;
public MyLinkedList() {
length =0;
this.head = new ListNode(0);
this.tail = new ListNode(0);
head.next= tail;
tail.prev= head;
}
public int get(int index) {
if(index < 0 || index >=length){
return -1;
}
ListNode cur = head;
for(int i = 0;i <= index;i++){
cur =cur.next;
}
return cur.val;
}
public void addAtHead(int val) {
addAtIndex(0,val);
}
public void addAtTail(int val) {
addAtIndex(length,val);
}
public void addAtIndex(int index, int val) {
if(index >length){
return ;
}
if(index<0){
index = 0;
}
ListNode cur= this.head;
for (int i = 0; i < index; i++) {
cur = cur.next;
}
ListNode newNode = new ListNode(val);
//原先A->B ---> A->newNode->B
// newNode->B newNode <-B
// A->newNode A <-newNode
newNode.next =cur.next;
cur.next.prev = newNode;
newNode.prev = cur;
cur.next =newNode;
length++;
}
public void deleteAtIndex(int index) {
if(index < 0 || index >= length){
return;
}
//长度超过一半，使用pre遍历
if(index > length/2){
ListNode cur =tail;
for (int i = 0; i <= length - index; i++) {
cur = cur.prev;
}
cur.next.next.prev = cur;
cur.next =cur.next.next;
}else{
ListNode cur =head;
for (int i = 0; i < index; i++) {
cur = cur.next;
}
cur.next.next.prev = cur;
cur.next =cur.next.next;
}
length--;
}
class ListNode{
int val;
ListNode prev,next;
ListNode(){
}
ListNode(int val){
this.val= val;
}
}
}
/**
* Your MyLinkedList object will be instantiated and called as such:
* MyLinkedList obj = new MyLinkedList();
* int param_1 = obj.get(index);
* obj.addAtHead(val);
* obj.addAtTail(val);
* obj.addAtIndex(index,val);
* obj.deleteAtIndex(index);
*/
双链表
class MyLinkedList {
int length;
ListNode head,tail;
public MyLinkedList() {
length =0;
this.head = new ListNode(0);
this.tail = new ListNode(0);
head.next= tail;
tail.prev= head;
}
public int get(int index) {
if(index < 0 || index >=length){
return -1;
}
ListNode cur = head;
for(int i = 0;i <= index;i++){
cur =cur.next;
}
return cur.val;
}
public void addAtHead(int val) {
addAtIndex(0,val);
}
public void addAtTail(int val) {
addAtIndex(length,val);
}
public void addAtIndex(int index, int val) {
if(index >length){
return ;
}
if(index<0){
index = 0;
}
ListNode cur= this.head;
for (int i = 0; i < index; i++) {
cur = cur.next;
}
ListNode newNode = new ListNode(val);
//原先A->B ---> A->newNode->B
// newNode->B newNode <-B
// A->newNode A <-newNode
newNode.next =cur.next;
cur.next.prev = newNode;
newNode.prev = cur;
cur.next =newNode;
length++;
}
public void deleteAtIndex(int index) {
if(index < 0 || index >= length){
return;
}
//长度超过一半，使用pre遍历
if(index > length/2){
ListNode cur =tail;
for (int i = 0; i <= length - index; i++) {
cur = cur.prev;
}
cur.next.next.prev = cur;
cur.next =cur.next.next;
}else{
ListNode cur =head;
for (int i = 0; i < index; i++) {
cur = cur.next;
}
cur.next.next.prev = cur;
cur.next =cur.next.next;
}
length--;
}
class ListNode{
int val;
ListNode prev,next;
ListNode(){
}
ListNode(int val){
this.val= val;
}
}
}
/**
* Your MyLinkedList object will be instantiated and called as such:
* MyLinkedList obj = new MyLinkedList();
* int param_1 = obj.get(index);
* obj.addAtHead(val);
* obj.addAtTail(val);
* obj.addAtIndex(index,val);
* obj.deleteAtIndex(index);
*/
