add_two.py

# 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]:
        return self.add_nodes(l1, l2)
    
    def add_nodes(self, l1, l2, prev_node=None, carried_over=0):
        total_sum = carried_over
        to_carry_over = 0
        next_node_1, next_node_2 = None, None
        if l1 or l2:
            if l1:
                total_sum += l1.val
                next_node_1 = l1.next
            if l2:
                total_sum += l2.val
                next_node_2 = l2.next
            if total_sum > 9:
                total_sum = total_sum - 10
                to_carry_over = 1
            if not prev_node:
                node = ListNode(val=total_sum, next=None)
            else:
                node = ListNode(val=total_sum, next=None)
                prev_node.next = node
            self.add_nodes(next_node_1, next_node_2, node, to_carry_over)
            return node
        else:
            if carried_over > 0:
                prev_node.next = ListNode(val=carried_over, next=None)