Merge two sorted lists

leetcode.cn

# 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]:

        if not list1: return list2
        if not list2: return list1

        if list1.val <= list2.val: 
            list1.next = self.mergeTwoLists(list1.next,list2)
            return list1 
        else: 
            list2.next = self.mergeTwoLists(list1,list2.next)
            return list2