Merge Two Sorted Lists

2    def mergeTwoLists(self, list1, list2):
●        # Create dummy node to simplify edge cases
4        dummy = ListNode(-1)
5        current = dummy
6 
7        # Merge while both lists have nodes
8        while list1 and list2:
9            if list1.val <= list2.val:
10                current.next = list1
11                list1 = list1.next
12            else:
......
15            current = current.next
16 
17        # Attach remaining nodes
18        if list1:
19            current.next = list1