# 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 list1 is None:
            return list2
        if list2 is None:
            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

# class Solution:
#     def mergeTwoLists(self, list1: Optional[ListNode], list2: Optional[ListNode]) -> Optional[ListNode]:
#         result = ListNode(-1)
#         dummy = result
#         while list1 and list2:
#             if list1.val < list2.val:
#                 dummy.next = list1
#                 list1 = list1.next
#             else:
#                 dummy.next = list2
#                 list2 = list2.next
#             dummy = dummy.next
        
#         if list1:
#             dummy.next = list1
#         else:
#             dummy.next = list2
    
#         return result.next
