1. # Definition for singly-linked list.
    2. # class ListNode:
    3. #     def __init__(self, x):
    4. #         self.val = x
    5. #         self.next = None
    7. class Solution:
    8.     def addTwoNumbers(self, l1: ListNode, l2: ListNode) -> ListNode:
    9.         ans = ListNode(0)
    10.         dummy = ans
    11.         carry = 0
    12.         while l1 or l2 or carry:
    13.             if l1 and l2:
    14.                 he = l1.val + l2.val + carry
    15.                 l1 = l1.next
    16.                 l2 = l2.next
    17.             elif l1 and not l2:
    18.                 he = l1.val + carry
    19.                 l1 = l1.next
    20.             elif l2 and not l1:
    21.                 he = l2.val + carry
    22.                 l2 = l2.next
    23.             else:
    24.                 he = carry
    25.             node_val = he % 10
    26.             carry = he // 10
    27.             dummy.next = ListNode(node_val)
    28.             dummy = dummy.next
    30.         return ans.next