# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, val=0, next=None):
#         self.val = val
#         self.next = next
ZERO = ListNode()

class Solution:
    def addTwoNumbers(self, l1: ListNode, l2: ListNode) -> ListNode:
        head = tail = None

        carry = 0
        while l1 or l2:
            d = (l1 or ZERO).val + (l2 or ZERO).val + carry
            carry = d // 10
            d %= 10

            l1 = (l1 or ZERO).next
            l2 = (l2 or ZERO).next

            if not head:
                head = tail = ListNode(d)
            else:
                tail.next = tail = ListNode(d)

        if carry != 0:
            tail.next = ListNode(carry)

        return head