# 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]:
        def reverseList(head):
            prev, curr = None, head
            while curr:
                temp = curr.next
                curr.next = prev
                prev = curr
                curr = temp
            return prev

        l1 = reverseList(l1)
        l2 = reverseList(l2)
        head = None
        carry = 0

        while l1 or l2 or carry:
            v1 = l1.val if l1 else 0
            v2 = l2.val if l2 else 0
            total = v1 + v2 + carry
            carry = total // 10
            node = ListNode(total % 10)
            node.next = head
            head = node
            l1 = l1.next if l1 else None
            l2 = l2.next if l2 else None

        return head

# 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]:
        s1, s2 = [], []

        while l1:
            s1.append(l1.val)
            l1 = l1.next

        while l2:
            s2.append(l2.val)
            l2 = l2.next

        carry = 0
        head = None

        while s1 or s2 or carry:
            v1 = s1.pop() if s1 else 0
            v2 = s2.pop() if s2 else 0
            total = v1 + v2 + carry
            carry = total // 10
            node = ListNode(total % 10)
            node.next = head
            head = node

        return head