class ListNode:
def __init__(self, x):
self.val = x
self.next = None
class MyLinkedList:
def __init__(self):
self.size = 0
self.head = ListNode(0) # sentinel node as pseudo-head
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 index is invalid
if index < 0 or index >= self.size:
return -1
head = self.head
# index steps needed
# to move from sentinel node to wanted index
for _ in range(index + 1):
head = head.next
return head.val
def addAtHead(self, val: int) -> None:
"""
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.
"""
self.addAtIndex(0, val)
def addAtTail(self, val: int) -> None:
"""
Append a node of value val to the last element of the linked list.
"""
self.addAtIndex(self.size, val)
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 is greater than the length,
# the node will not be inserted.
if index > self.size:
return
# [so weird] If index is negative,
# the node will be inserted at the head of the list.
if index < 0:
index = 0
self.size += 1
# find predecessor of the node to be added
pred = self.head
for _ in range(index):
pred = pred.next
# node to be added
to_add = ListNode(val)
# insertion itself
to_add.next = pred.next
pred.next = to_add
def deleteAtIndex(self, index: int) -> None:
"""
Delete the index-th node in the linked list, if the index is valid.
"""
# if the index is invalid, do nothing
if index < 0 or index >= self.size:
return
self.size -= 1
# find predecessor of the node to be deleted
pred = self.head
for _ in range(index):
pred = pred.next
# delete pred.next
pred.next = pred.next.next
