[链表]合并两个有序链表

# 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]:

        first = list1
        second = list2

        dummy = ListNode(-1)
        tail = dummy

        while first and second:
            if first.val <= second.val:
                tail.next = first
                first = first.next
            else:
                tail.next = second
                second = second.next

            tail = tail.next
            tail.next = None
        
        if first:
            tail.next = first
        elif second:
            tail.next = second

        return dummy.next