21.py

# 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]:
        list3 = ListNode(0)
        sts = list3
        while list1 and list2:
            if list1.val <= list2.val:
                list3.next = list1
                list3 = list3.next
                list1 = list1.next
                # print(list3,list1)
            else:
                list3.next = list2
                list3 = list3.next
                list2 = list2.next   
                # print(list3,list2)           
        if list1 == None:
            list3.next = list2
        if list2 == None:
            list3.next = list1
        return sts.next