707. Design linked list (linked list)

subject ：

Implement these functions in the linked list class ：

• get(index)： Get the  index  Values of nodes . If the index is invalid , Then return to -1.
• addAtHead(val)： Add a value of... Before the first element of the list  val  The node of . After inserting , The new node will be the first node in the list .
• addAtTail(val)： Will be worth  val To the last element of the list .
• addAtIndex(index,val)： In the list, the  index  The value added before nodes is  val  The node of . If  index  It's equal to the length of the list , Then the node will be attached to the end of the list . If index Greater than the length of the list , The node will not be inserted . If index Less than 0, Then insert the node in the head .
• deleteAtIndex(index)： If the index  index It works , Delete the  index Nodes .

public class Shejilianbiao {
    // Define linked list 
    static class ListNode{
        int val; // data ： Node data 
        ListNode next; // object ： Reference next data object 

        // Define construction method 
        ListNode(int val){
            this.val = val;
        }
    }
    static class MyLinkedList{
        //size Stores the number of linked list elements 
        int size;
        // Virtual header node 
        ListNode head;

        // Initialize linked list 
        public MyLinkedList(){
            size = 0;
            head = new ListNode(0);
        }

        // For the first index The number of nodes 
        public int get(int index){
            // If index illegal , return -1
            if(index< 0 || index>=size){
                return -1;
            }
            ListNode currentNode = head;
            // Use for loop , Find the first (index+1) Nodes 
            for (int i = 0; i <=index ; i++) {
                currentNode = currentNode.next;
            }
            return currentNode.val;
        }

        // Insert a node at the front of the linked list 
        public void addAtHead(int val){
            addAtIndex(0,val);
        }

        // Insert a node at the end of the linked list 
        public  void addAtTail(int val){
            addAtIndex(size,val);
        }

        // In the index Insert a new node before two nodes , for example index by 0, Then the newly inserted node is the new head node of the linked list .
        // If index It's equal to the length of the list , The newly inserted node is the tail node of the linked list 
        // If index Greater than the length of the list , It returns null 
        public void addAtIndex(int index,int val){
            if(index>size){
                return;
            }
            if (index<0){
                index=0;
            }// Insert node , Add one to the length of the linked list 
            size++;
            // The precursor of the node to be inserted 
            ListNode pre = head;
            for (int i = 0; i <index ; i++) {
                pre = pre.next;
            }
            // Create a new node to insert 
            ListNode add = new ListNode(val);
            // The pointer to the inserted section points to the next node of the previous node 
            add.next = pre.next;
            // The previous node points to the insertion node 
            pre.next = add;
        }

        // Delete the first index Nodes 
        public  void deleteAtIndex(int index){
            // If index Out of range , Returns an empty 
            if(index < 0 || index >=size ){
                return;
            }
            // Delete node , Total length minus one 
            size--;
            ListNode pre = head;
            for (int i = 0; i <index ; i++) {
                pre = pre.next;
            }
            pre.next = pre.next.next;
        }
    }

    public static void main(String[] args) {
        MyLinkedList myLinkedList = new MyLinkedList();
        myLinkedList.addAtHead(1);
        myLinkedList.addAtTail(3);
        myLinkedList.addAtIndex(1,2);
        int x = myLinkedList.get(1);
        myLinkedList.deleteAtIndex(1);
        int y = myLinkedList.get(1);
        System.out.println(x+","+y);
    }
}

Output results ：

2,3