leetcode 21.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]:
        if list1 == None and list2 == None:
            return None
        elif list1==None and list2!=None:
            return list2
        elif list1!=None and list2==None:
            return list1
        elif (list1.val<=list2.val):
            head = list1
            list1= list1.next
        else:
            head = list2
            list2= list2.next
        
        walk = head
        while(list1!=None and list2!=None):
            if list1.val <= list2.val:
                walk.next = list1
                walk = walk.next
                list1 = list1.next
            else:
                walk.next = list2
                walk = walk.next
                list2 = list2.next
        if list1!=None:
            walk.next = list1
        elif list2 != None:
            walk.next = list2
        else:
            walk.next = None
        
        return head