21.合并2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]:
        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:
            tail.next =list1
        elif list2:
            tail.next=list2 
        return dummy.next