# 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]:
        i = 1
        n1 = n2 = 0
        while not (l1 == None and l2 == None):
            if l1 != None:
                n1 += (l1.val * i)
                l1 = l1.next
            if l2 != None:
                n2 += (l2.val * i)
                l2 = l2.next
            i *= 10
        n = None
        for c in list(str(n1 + n2)):
            n = ListNode(int(c), n)
        return n