设计链表的实现。您可以选择使用单链表或双链表。单链表中的节点应该具有两个属性：val 和 next。val 是当前节点的值，next 是指向下一个节点的指针/引用。如果要使用双向链表，则还需要一个属性 prev 以指示链表中的上一个节点。假设链表中的所有节点都是 0-index 的。
在链表类中实现这些功能：
get(index)：获取链表中第 index 个节点的值。如果索引无效，则返回-1。
addAtHead(val)：在链表的第一个元素之前添加一个值为 val 的节点。插入后，新节点将成为链表的第一个节点。
addAtTail(val)：将值为 val 的节点追加到链表的最后一个元素。
addAtIndex(index,val)：在链表中的第 index 个节点之前添加值为 val 的节点。如果 index 等于链表的长度，则该节点将附加到链表的末尾。如果 index 大于链表长度，则不会插入节点。如果index小于0，则在头部插入节点。
deleteAtIndex(index)：如果索引 index 有效，则删除链表中的第 index 个节点。
示例：
MyLinkedList linkedList = new MyLinkedList();
linkedList.addAtHead(1);
linkedList.addAtTail(3);
linkedList.addAtIndex(1,2); //链表变为1-> 2-> 3
linkedList.get(1); //返回2
linkedList.deleteAtIndex(1); //现在链表是1-> 3
linkedList.get(1); //返回3
提示：
所有val值都在 [1, 1000] 之内。
操作次数将在 [1, 1000] 之内。
请不要使用内置的 LinkedList 库。
方法一：
# 单链表
class Node:
def __init__(self, val):
self.val = val
self.next = None
class MyLinkedList:
def __init__(self):
self._head = Node(0) # 虚拟头部节点
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:
node = self._head
for _ in range(index + 1):
node = node.next
return node.val
else:
return -1
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._count, 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 < 0:
index = 0
elif index > self._count:
return
# 计数累加
self._count += 1
add_node = Node(val)
prev_node, current_node = None, self._head
for _ in range(index + 1):
prev_node, current_node = current_node, current_node.next
else:
prev_node.next, add_node.next = add_node, current_node
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:
# 计数-1
self._count -= 1
prev_node, current_node = None, self._head
for _ in range(index + 1):
prev_node, current_node = current_node, current_node.next
else:
prev_node.next, current_node.next = current_node.next, None
方法二：
# 双链表
# 相对于单链表, Node新增了prev属性
class Node:
def __init__(self, val):
self.val = val
self.prev = None
self.next = None
class MyLinkedList:
def __init__(self):
self._head, self._tail = Node(0), Node(0) # 虚拟节点
self._head.next, self._tail.prev = self._tail, self._head
self._count = 0 # 添加的节点数
def _get_node(self, index: int) -> Node:
# 当index小于_count//2时, 使用_head查找更快, 反之_tail更快
if index >= self._count // 2:
# 使用prev往前找
node = self._tail
for _ in range(self._count - index):
node = node.prev
else:
# 使用next往后找
node = self._head
for _ in range(index + 1):
node = node.next
return node
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:
node = self._get_node(index)
return node.val
else:
return -1
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._update(self._head, self._head.next, val)
def addAtTail(self, val: int) -> None:
"""
Append a node of value val to the last element of the linked list.
"""
self._update(self._tail.prev, self._tail, 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 < 0:
index = 0
elif index > self._count:
return
node = self._get_node(index)
self._update(node.prev, node, val)
def _update(self, prev: Node, next: Node, val: int) -> None:
"""
更新节点
:param prev: 相对于更新的前一个节点
:param next: 相对于更新的后一个节点
:param val: 要添加的节点值
"""
# 计数累加
self._count += 1
node = Node(val)
prev.next, next.prev = node, node
node.prev, node.next = prev, next
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:
node = self._get_node(index)
# 计数-1
self._count -= 1
node.prev.next, node.next.prev = node.next, node.prev
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