text: # https://leetcode.com/problems/merge-two-sorted-lists/
  
  # 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]:
          dummy = ListNode()
          tail = dummy
          
          while list1 and list2:
              if list1.val < list2.val:
                  tail.next = list1
                  list1 = list1.next
              else:
                  tail.next = list2
                  list2 = list2.next
              tail = tail.next
              
          if list1 == None and list2 != None:
              tail.next = list2
          elif list1 != None and list2 == None:
              tail.next = list1
              
          return dummy.next
          
  