### Description#

Design your implementation of the linked list. You can choose to use a singly or 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.

Implement the `MyLinkedList` class:

• `MyLinkedList()` Initializes the `MyLinkedList` object.
• `int get(int index)` Get the value of the `indexth` node in the linked list. If the index is invalid, return `-1`.
• `void addAtHead(int 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.
• `void addAtTail(int val)` Append a node of value `val` as the last element of the linked list.
• `void addAtIndex(int index, int val)` Add a node of value `val` before the `indexth` node in the linked list. If `index` equals the length of the linked list, the node will be appended to the end of the linked list. If `index` is greater than the length, the node will not be inserted.
• `void deleteAtIndex(int index)` Delete the `indexth` node in the linked list, if the index is valid.

### Solution#

``````class Node:

def __init__(self, val):
self.val = val
self.next = None

def __init__(self):
self._count = 0  # 添加的节点数

def get(self, index: int) -> int:
"""
Get the value of the index-th node in the linked list. If the index is invalid, return -1.
"""
if 0 <= index < self._count:
for _ in range(index + 1):
node = node.next
return node.val
else:
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.
"""

def addAtTail(self, val: int) -> None:
"""
Append a node of value val to the last element of the linked list.
"""

def addAtIndex(self, index: int, val: int) -> None:
"""
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.
"""
if index < 0:
index = 0
elif index > self._count:
return

# number of nodes + 1
self._count += 1

for _ in range(index + 1):
prev_node, current_node = current_node, current_node.next
else:

def deleteAtIndex(self, index: int) -> None:
"""
Delete the index-th node in the linked list, if the index is valid.
"""
if 0 <= index < self._count:
# counts -1
self._count -= 1