添加链接描述

# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, val=0, next=None):
#         self.val = val
#         self.next = next
class Solution:def mergeTwoLists(self, list1: Optional[ListNode], list2: Optional[ListNode]) -> Optional[ListNode]:# 思路是使用递归# 递归出条件也是先踢出的条件：if not list1:return list2if not list2:return list1if list1.val<=list2.val:list1.next=self.mergeTwoLists(list1.next,list2)return list1else:list2.next=self.mergeTwoLists(list1,list2.next)return list2

思路：

1. 递归，合并同类
2. list1.next=self.mergeTwoLists(list1.next,list2)这里因为要递归，在雷里面调用函数，需要self