要点⭐
链表的常见操作
获取第n个节点的值
头部插入节点
尾部插入节点
第n个节点前插入（先立新，在破旧）
删除第n个节点

class ListNode{int val;ListNode next;ListNode(){};ListNode(int val){this.val=val;}
}
class MyLinkedList {//链表大小int size;//虚拟头节点ListNode head;//初始化链表public MyLinkedList(){size=0;head=new ListNode(0);}public int get(int index) {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++;}public void addAtTail(int val) {ListNode newNode=new ListNode(val);ListNode currentNode=head;while(currentNode.next!=null){currentNode=currentNode.next;}currentNode.next=newNode;size++;}public void addAtIndex(int index, int val) {if(index>size){return;}if(index<0){index=0;}size++;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;}public void deleteAtIndex(int index) {if(index<0||index>=size){return;}size--;ListNode pred=head;for(int i=0;i<index;i++){pred=pred.next;}pred.next=pred.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);*/