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]:
        # if not (list1 or list2):
        #     return None
        # tmp = list()
        # while list1 and list2:
        #     if not list1 or list1.val > list2.val:
        #         tmp.append(list2.val)
        #         list2 = list2.next
        #     else:
        #         tmp.append(list1.val)
        #         list1 = list1.next
        # while list1:
        #     tmp.append(list1.val)
        #     list1 = list1.next
        # while list2:
        #     tmp.append(list2.val)
        #     list2 = list2.next
        #
        # ans = ListNode(val=tmp[-1])
        # for i in reversed(tmp[:-1]):
        #     ans = ListNode(val=i, next=ans)
        # return ans
        dummy = ListNode()
        tmp = dummy

        while list1 and list2:
            if list1.val <= list2.val:
                tmp.next = list1
                list1 = list1.next
            else:
                tmp.next = list2
                list2 = list2.next
            tmp = tmp.next

        tmp.next = list1 or list2
        return dummy.next