203. 移除链表元素
关键词：定义一个虚拟头节点
/**
* Definition for singly-linked list.
* public class ListNode {
* int val;
* ListNode next;
* ListNode() {}
* ListNode(int val) { this.val = val; }
* ListNode(int val, ListNode next) { this.val = val; this.next = next; }
* }
*/
class Solution {
public ListNode removeElements(ListNode head, int val) {
//定义一个虚拟头节点
ListNode virtualHead = new ListNode();
virtualHead.next = head;
ListNode cur = virtualHead;
while(cur.next!=null){
if(cur.next.val == val){
cur.next = cur.next.next;
}else{
cur = cur.next;
}
}
return virtualHead.next;
}
}
707. 设计链表
关键词：虚拟头节点，cur遍历节点（不停指向下一个实现遍历）
单链表
- 个人解法（11ms，和标准的10ms差不太多 没必要改）
class MyLinkedList {
int size;
//虚拟头节点
ListNode head;
public MyLinkedList() {
size = 0;
head = new ListNode(0);
}
//定义一个ListNode遍历链表
public int get(int index) {
if(index<0 || index >= size) return -1;
ListNode cur = head;
//i从0开始，0即头节点的位置
for(int i = 0; i<=index; i++){
cur = cur.next;
}
return cur.val;
}
public void addAtHead(int val) {
ListNode ele = new ListNode();
ele.val = val;
ele.next = head.next;
head.next = ele;
size++;
}
public void addAtTail(int val) {
ListNode cur = head;
ListNode ele = new ListNode();
ele.val = val;
for(int i=0; i<size; i++){
cur = cur.next;
}
cur.next = ele;
size++;
}
public void addAtIndex(int index, int val) {
ListNode ele = new ListNode();
ele.val = val;
if( index == size) {
addAtTail(val);
}else if(index < size){
ListNode cur = head;
for(int i=0; i<index; i++){
cur = cur.next;
}
ele.next = cur.next;
cur.next = ele;
size++;
}
}
public void deleteAtIndex(int index) {
ListNode cur = head;
if(index < size){
for(int i=0; i<index; i++){
cur = cur.next;
}
cur.next = cur.next.next;
size--;
}
}
}
- 标准解法
//单链表
class ListNode {
int val;
ListNode next;
ListNode(){}
ListNode(int val) {
this.val=val;
}
}
class MyLinkedList {
//size存储链表元素的个数
int size;
//虚拟头结点
ListNode head;
//初始化链表
public MyLinkedList() {
size = 0;
head = new ListNode(0);
}
//获取第index个节点的数值，注意index是从0开始的，第0个节点就是头结点
public int get(int index) {
//如果index非法，返回-1
if (index < 0 || index >= size) {
return -1;
}
ListNode currentNode = head;
//包含一个虚拟头节点，所以查找第 index+1 个节点
for (int i = 0; i <= index; i++) {
currentNode = currentNode.next;
}
return currentNode.val;
}
public void addAtHead(int val) {
ListNode newNode = new ListNode(val);
newNode.next = head.next;
head.next = newNode;
size++;
// 在链表最前面插入一个节点，等价于在第0个元素前添加
// addAtIndex(0, val);
}
public void addAtTail(int val) {
ListNode newNode = new ListNode(val);
ListNode cur = head;
while (cur.next != null) {
cur = cur.next;
}
cur.next = newNode;
size++;
// 在链表的最后插入一个节点，等价于在(末尾+1)个元素前添加
// addAtIndex(size, val);
}
// 在第 index 个节点之前插入一个新节点，例如index为0，那么新插入的节点为链表的新头节点。
// 如果 index 等于链表的长度，则说明是新插入的节点为链表的尾结点
// 如果 index 大于链表的长度，则返回空
public void addAtIndex(int index, int val) {
if (index > size) {
return;
}
if (index < 0) {
index = 0;
}
size++;
//找到要插入节点的前驱
ListNode pred = head;
for (int i = 0; i < index; i++) {
pred = pred.next;
}
ListNode toAdd = new ListNode(val);
toAdd.next = pred.next;
pred.next = toAdd;
}
//删除第index个节点
public void deleteAtIndex(int index) {
if (index < 0 || index >= size) {
return;
}
size--;
//因为有虚拟头节点，所以不用对Index=0的情况进行特殊处理
ListNode pred = head;
for (int i = 0; i < index ; i++) {
pred = pred.next;
}
pred.next = pred.next.next;
}
}
//双链表
class ListNode{
int val;
ListNode next,prev;
ListNode() {};
ListNode(int val){
this.val = val;
}
}
class MyLinkedList {
//记录链表中元素的数量
int size;
//记录链表的虚拟头结点和尾结点
ListNode head,tail;
public MyLinkedList() {
//初始化操作
this.size = 0;
this.head = new ListNode(0);
this.tail = new ListNode(0);
//这一步非常关键，否则在加入头结点的操作中会出现null.next的错误！！！
head.next=tail;
tail.prev=head;
}
public int get(int index) {
//判断index是否有效
if(index>=size){
return -1;
}
ListNode cur = this.head;
//判断是哪一边遍历时间更短
if(index >= size / 2){
//tail开始
cur = tail;
for(int i=0; i< size-index; i++){
cur = cur.prev;
}
}else{
for(int i=0; i<= index; i++){
cur = cur.next;
}
}
return cur.val;
}
public void addAtHead(int val) {
//等价于在第0个元素前添加
addAtIndex(0,val);
}
public void addAtTail(int val) {
//等价于在最后一个元素(null)前添加
addAtIndex(size,val);
}
public void addAtIndex(int index, int val) {
//index大于链表长度
if(index>size){
return;
}
size++;
//找到前驱
ListNode pre = this.head;
for(int i=0; i<index; i++){
pre = pre.next;
}
//新建结点
ListNode newNode = new ListNode(val);
newNode.next = pre.next;
pre.next.prev = newNode;
newNode.prev = pre;
pre.next = newNode;
}
public void deleteAtIndex(int index) {
//判断索引是否有效
if(index>=size){
return;
}
//删除操作
size--;
ListNode pre = this.head;
for(int i=0; i<index; i++){
pre = pre.next;
}
pre.next.next.prev = pre;
pre.next = pre.next.next;
}
}
/**
* 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);
*/
206. 反转链表
关键词：链表最主要的就是定义节点下一指向，实现反转可以在不用新创建链表的情况下只用指针控制
- 双指针写法
/**
* Definition for singly-linked list.
* public class ListNode {
* int val;
* ListNode next;
* ListNode() {}
* ListNode(int val) { this.val = val; }
* ListNode(int val, ListNode next) { this.val = val; this.next = next; }
* }
*/
class Solution {
public ListNode reverseList(ListNode head) {
//双指针写法
ListNode cur = head;
ListNode pre = null;
while(cur!=null){
ListNode temp = cur.next;
cur.next = pre;
pre = cur;
cur = temp;
}
return pre;
}
}
- 递归写法
class Solution {
//递归写法
public ListNode reverseList(ListNode head) {
return reverse(head, null);
}
public ListNode reverse(ListNode cur, ListNode pre) {
if(cur == null) return pre;
ListNode temp = cur.next;
cur.next = pre;
return reverse(temp, cur);
}
}
