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);
 */