1class MyLinkedList {
2    class Node {
3        int val;
4        Node next;
5        Node(int val) { this.val = val; }
6    }
7    
8    private Node head;
9
10    public MyLinkedList() {
11        head = null;
12    }
13
14    public int get(int index) {
15        Node current = head;
16        for (int i = 0; current != null && i < index; i++) {
17            current = current.next;
18        }
19        return current == null ? -1 : current.val;
20    }
21
22    public void addAtHead(int val) {
23        Node node = new Node(val);
24        node.next = head;
25        head = node;
26    }
27
28    public void addAtTail(int val) {
29        Node node = new Node(val);
30        if (head == null) {
31            head = node;
32            return;
33        }
34        Node current = head;
35        while (current.next != null) {
36            current = current.next;
37        }
38        current.next = node;
39    }
40
41    public void addAtIndex(int index, int val) {
42        if (index == 0) {
43            addAtHead(val);
44            return;
45        }
46        Node current = head;
47        for (int i = 0; current != null && i < index - 1; i++) {
48            current = current.next;
49        }
50        if (current == null) return;
51        Node node = new Node(val);
52        node.next = current.next;
53        current.next = node;
54    }
55
56    public void deleteAtIndex(int index) {
57        if (index == 0 && head != null) {
58            head = head.next;
59            return;
60        }
61        Node current = head;
62        for (int i = 0; current != null && i < index - 1; i++) {
63            current = current.next;
64        }
65        if (current != null && current.next != null) {
66            current.next = current.next.next;
67        }
68    }
69}

1
    private class Node {
        public int val;
        public Node next;
        public Node prev;
        public Node(int _val) { val = _val; }
    }

    private Node head;

    public MyLinkedList() {
        head = null;
    }

    public int Get(int index) {
        Node current = head;
        for (int i = 0; i < index && current != null; i++) {
            current = current.next;
        }
        return current == null ? -1 : current.val;
    }

    public void AddAtHead(int val) {
        Node node = new Node(val);
        node.next = head;
        if (head != null) head.prev = node;
        head = node;
    }

    public void AddAtTail(int val) {
        Node node = new Node(val);
        if (head == null) {
            head = node;
        } else {
            Node current = head;
            while (current.next != null) {
                current = current.next;
            }
            current.next = node;
            node.prev = current;
        }
    }
    
    public void AddAtIndex(int index, int val) {
        if (index == 0) {
            AddAtHead(val);
            return;
        }
        Node current = head;
        for (int i = 0; i < index - 1 && current != null; i++) {
            current = current.next;
        }
        if (current != null) {
            Node node = new Node(val);
            node.next = current.next;
            if (current.next != null) current.next.prev = node;
            current.next = node;
            node.prev = current;
        }
    }

    public void DeleteAtIndex(int index) {
        Node current = head;
        for (int i = 0; current != null && i < index; i++) {
            current = current.next;
        }
        if (current != null) {
            if (current.prev != null) current.prev.next = current.next;
            else head = current.next;
            if (current.next != null) current.next.prev = current.prev;
        }
    }
}