add-two-numbers.py

class Solution:
    def addTwoNumbers(self, l1, l2):
        if l1 is None:
            return l2

        if l2 is None:
            return l1

        ptr1 = l1
        ptr2 = l2
        result = ListNode(None)
        prev = result
        overflow = 0

        while ptr1 or ptr2 or overflow:
            val1 = ptr1.val if ptr1 else 0
            val2 = ptr2.val if ptr2 else 0

            val_sum = (val1 + val2 + overflow) % 10
            overflow = int((val1 + val2 + overflow) / 10)

            prev.next = ListNode(val_sum)
            prev = prev.next

            ptr1 = ptr1.next if ptr1 else None
            ptr2 = ptr2.next if ptr2 else None

        return result.next