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.

Example:

``````MyLinkedList linkedList = new MyLinkedList();
``````

Note:

• All values will be in the range of`[1, 1000]`.
• The number of operations will be in the range of `[1, 1000]`.

## Analysis

Easy题，但是一次做对还挺难的...

## Solution

Java (118ms)

``````class MyLinkedList {
class Node {
int val;
Node next;
public Node(int val) {
this.val = val;
}
}

private Node tail;
private int size;

/** Initialize your data structure here. */
tail = null;
size = 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 >= size) {
return -1;
}
for (int i = 0; i < index; i++) {
p = p.next;
}
return p.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. */
Node n = new Node(val);
size++;

}
}

/** Append a node of value val to the last element of the linked list. */
Node n = new Node(val);
if (tail != null) {
tail.next = n;
}
if (tail == null || size == 0) {
}
tail = n;
size++;
}

/** 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 || index > size) {
return;
}
if (index == 0) {

} else if (index == size) {

} else {
for (int i = 0; i < index - 1; i++) {
p = p.next;
}
/*
Node n = new Node(val);
n.next = p.next;
p.next = n;
*/
Node n = new Node(val);
Node save = p;
p = p.next;
save.next = n;
n.next = p;
size++;
}
}

/** Delete the index-th node in the linked list, if the index is valid. */
public void deleteAtIndex(int index) {
if (size == 0 || index < 0 || index >= size) {
return;
}
if (index == 0) {
size--;
if (size == 0) {
tail = null;
}
} else {
for (int i = 0; i < index - 1; i++) {
p = p.next;
}
p.next = p.next.next;
size--;
if (p.next == null) {
tail = p;
}
}
}
}

/**