第一题 https://leetcode.cn/problems/remove-linked-list-elements/submissions/691848826/
# Definition for singly-linked list. class ListNode: def __init__(self, val=0, next=None): self.val = val self.next = next from typing import Optional class Solution: def removeElements(self, head: Optional[ListNode], val: int) -> Optional[ListNode]: dummy = ListNode(0, head) cur = dummy while cur.next: if cur.next.val == val: cur.next = cur.next.next else: cur = cur.next return dummy.next注意cur.next为待删除目标时cur不往后移，因为后面还可能是相同元素。虚拟头结点还是挺方便的。
第二题 反转链表https://leetcode.cn/problems/reverse-linked-list/description/
def reverseList(self, head: Optional[ListNode]) -> Optional[ListNode]: cur = head pre = None while cur: next = cur.next cur.next = pre pre = cur cur = next return pre代码不难，也用不着虚拟头结点。
第三题 https://leetcode.cn/problems/design-linked-list/description/
设计类的还是有点难度的，注意索引是否越界要提前判断，首尾插入都应该是按索引插入的特例
