# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, val=0, next=None):
#         self.val = val
#         self.next = next

#iterative: time O(n), space O(n)
class Solution:
    def mergeTwoLists(self, list1: Optional[ListNode], list2: Optional[ListNode]) -> Optional[ListNode]:
      if not list1: return list2
      if not list2: return list1
      newList = cur = ListNode(0)
      while list1 and list2:
        if  list1.val < list2.val:
          cur.next = list1
          list1 = list1.next
        else:
          cur.next = list2
          list2 = list2.next
        cur = cur.next
      cur.next = list1 or list2
      return newList.next
