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class ListNode:
def __init__(self, x):
self.val = x
self.next = None
self.prev = None
class MyLinkedList:
def __init__(self):
"""
Initialize your data structure here.
"""
self.size = 0
self.head = ListNode(0)
self.tail = ListNode(0)
self.head.next = self.tail
self.tail.prev = self.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 < 0 or index >= self.size:
return -1
if index + 1 < self.size - index:
curr = self.head
for _ in range(index + 1):
curr = curr.next
else:
curr = self.tail
for _ in range(self.size - index):
curr = curr.prev
return curr.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 > self.size:
return
if index < 0:
index = 0
if index < self.size - index:
prev = self.head
for _ in range(index):
prev = prev.next
next = prev.next
else:
next = self.tail
for _ in range(self.size - index):
next = next.prev
prev = next.prev
self.size += 1
add_node = ListNode(val)
add_node.prev = prev
add_node.next = next
prev.next = add_node
next.prev = add_node
def deleteAtIndex(self, index: int) -> None:
"""
Delete the index-th node in the linked list, if the index is valid.
"""
if index < 0 or index >= self.size:
return
if index < self.size - index:
prev = self.head
for _ in range(index):
prev = prev.next
next = prev.next.next
else:
next = self.tail
for _ in range(self.size - index - 1):
next = next.prev
prev = next.prev.prev
self.size -= 1
prev.next = next
next.prev = prev
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