设计链表

class MyLinkedList {
    
public static class ListNode {
    private int val;
    private ListNode next;

    public ListNode(int val) {
      this.val = val;
    }
  }
    /** Initialize your data structure here. */
    public MyLinkedList() {
        
    }
    
    private ListNode listNode=null;
    
    /** Get the value of the index-th node in the linked list. If the index is invalid, return -1. */
    public int get(int index) {
        if(listNode==null){
            return -1;
        }
        ListNode itr=this.listNode;
        int count=0;
        while(itr!=null){
            if(count==index){
                return itr.val;
            }
            count++;
            itr=itr.next;
        }
        return -1;
    }
    
    /** Add a node of value val before the first element of the linked list. After the insertion, the new node will be the first node of the linked list. */
    public void addAtHead(int val) {
        ListNode newNode=new ListNode(val);
        newNode.next=this.listNode;
        this.listNode=newNode;
    }
    
    /** Append a node of value val to the last element of the linked list. */
    public void addAtTail(int val) {
        ListNode itr=this.listNode;
        ListNode last=null;
        ListNode newNode=new ListNode(val);
        while(itr!=null){
            if(itr.next==null){
                last=itr;
            }
            itr=itr.next;
        }
        if(last!=null){
            last.next=newNode;
        }else{
            this.listNode=newNode;
        }
    }
    
    /** Add a node of value val before the index-th node in the linked list. If index equals to the length of linked list, the node will be appended to the end of linked list. If index is greater than the length, the node will not be inserted. */
    public void addAtIndex(int index, int val) {
        if(index<=0){
            addAtHead(val);
        }else{
         ListNode itr=this.listNode;
        int count=0;
        ListNode newNode=new ListNode(val);
        while(itr!=null){
            if(count+1==index){
                ListNode tmp=itr.next;
                itr.next=newNode;
                if(tmp!=null){
                    newNode.next=tmp;
                }
                break;
            }
            count++;
            itr=itr.next;
        }   
        }
    }
    
    /** Delete the index-th node in the linked list, if the index is valid. */
    public void deleteAtIndex(int index) {
        ListNode itr=this.listNode;
        int count=0;
        ListNode prev=null;
        while(itr!=null){
            if(count==index){
                ListNode tmp=itr.next;
                if(prev!=null){
                 prev.next=tmp;   
                }else{
                 this.listNode=tmp;
                }
                break;
            }
            count++;
            prev=itr;
            itr=itr.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);
 */