题目：

题解：

# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, val=0, next=None):
#         self.val = val
#         self.next = next
class Solution:
    def addTwoNumbers(self, l1: Optional[ListNode], l2: Optional[ListNode]) -> Optional[ListNode]:
        carry = 0
        p = l1
        q = l2
        dummy = ListNode(0)
        k = dummy
        while p or q:
            val1 = p.val if p else 0
            val2 = q.val if q else 0
            n = val1 + val2 + carry
            carry = n // 10
            res = n % 10
            node = ListNode(res)
            k.next = node
            k = k.next
            if p:
                p = p.next
            if q:
                q = q.next
        if carry:
            node = ListNode(carry)
            k.next = node
        return dummy.next