Definition for singly-linked list.

# Definition for singly-linked list.# class ListNode:#     def __init__(self, val=0, next=None):#         self.val = val#         self.next = nextclass Solution:    def _add(self, l1, l2, answer, carry: int) -> Optional[ListNode]:        if not l1 and not l2 and carry == 0:            return answer        answer.val += l1.val if l1 else 0        answer.val += l2.val if l2 else 0        answer.val += carry        carry = 0        if answer.val >= 10:            answer.val -= 10            carry = 1        l1next = l1.next if l1 else None        l2next = l2.next if l2 else None        if l1next or l2next or carry != 0:            # print(l1next.val if l1next else 0, l2next.val if l2next else 0, carry)            n = ListNode()            answer.next = n        else:            n = None        self._add(l1next, l2next, n, carry)
    def addTwoNumbers(self, l1: Optional[ListNode], l2: Optional[ListNode]) -> Optional[ListNode]:        answer = ListNode()        self._add(l1, l2, answer, 0)        return answer