Hi geeks, in this article we going to design the Linked List with add at head, add at tail, insert at specific position and delete operations. This is one of the good question from Leetcode to test the LinkedList knowledge.

Design your implementation of the linked list. You can choose to use the singly linked list or the doubly linked list. A node in a singly linked list should have two attributes: `val` and `next``val` is the value of the current node, and `next` is a pointer/reference to the next node. If you want to use the doubly linked list, you will need one more attribute `prev` to indicate the previous node in the linked list. Assume all nodes in the linked list are 0-indexed.

• `get(index)` : Get the value of the `index`-th node in the linked list. If the index is invalid, return `-1`.
• `addAtHead(val)` : 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.
• `addAtTail(val)` : Append a node of value `val` to the last element of the linked list.
• `addAtIndex(index, val)` : 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.
• `deleteAtIndex(index)` : Delete the `index`-th node in the linked list, if the index is valid.
```Input:
[[],,,[1,2],,,]
Output:
[null,null,null,null,2,null,3] ```

### Solution:

For this solution, I used a singly linked list with a sentinel node. The sentinel node is a nice way of avoiding edge cases. It makes the code much simpler. I also keep a pointer to the last element (`tailPred`) to allow fast insertion at the tail.

To avoid redundancy, I added two helper methods, `findPred` and `addAfter`. This keeps the code clean.

```/**
* A singly linked list with a left sentinel node.
*/

/** A very simple node class. */
private static class Node {
int val;
Node next;
}

// Predecessor of the first element
// Predecessor of the tail
private Node tailPred;
private int length;

/** Initialize your data structure here. */
length = 0;
}

/** 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 ((index < 0) || (index >= length)) {
return -1;
}
return findPred(index).next.val;
}

/** 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. */
if (length == 0) {
} else {
}
}

/** Append a node of value val to the last element of the linked list. */
tailPred = tailPred.next;
}

/** 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) {
} else if (index == length) {
} else if ((index >= 0) && (index < length)) {
}
}

/** Delete the index-th node in the linked list, if the index is valid. */
public void deleteAtIndex(int index) {
if ((index >= 0) && (index < length)) {
Node pred = findPred(index);
if (index == length - 1) { // Remove last element
// Move tail to the left
tailPred = pred;
}
pred.next = pred.next.next;
--length;
}
}

/** Return the predecessor of the index-th node. */
private Node findPred(int index) {
for (int i = 0; i < index; ++i) {
pred = pred.next;
}
return pred;
}

/** Add an element after the given node. */
private void addAfter(Node pred, int val) {
Node node = new Node();
node.val = val;
node.next = pred.next;
pred.next = node;
++length;
}
}
```