Add Two Numbers

LeetCode 2

# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, x):
#         self.val = x
#         self.next = None

class Solution:
    def addTwoNumbers(self, l1, l2):
        """
        :type l1: ListNode
        :type l2: ListNode
        :rtype: ListNode
        """
        dummy = node =ListNode('#')
        carry = 0
        # step 1: tried to l1 l2 move
        # step 4: what if l1 and l2 are None but carry is out there?
        # add "carry" to the condition of while.
        while l1 or l2 or carry:
            v1, v2 = 0, 0
            if l1:
                v1 = l1.val
                l1 = l1.next
            if l2:
                v2 = l2.val
                l2 = l2.next
            # step 2: carry variable is needed
            carry, cursum = divmod(carry+v1+v2, 10)
            # step 3: a dummy node is needed.
            node.next = ListNode(cursum)
            node = node.next
        return dummy.next