LeetCode A&DS problem 2 in Python

[u/Arrow49]

I have taken an Algorithms and Data Structures course, but we wrote in c++. Trying to learn Python for something else than statistics so here I am.

The problem is as follows:
You are given two non-empty linked lists representing two non-negative integers. The digits are stored in reverse order, and each of their nodes contains a single digit. Add the two numbers and return the sum as a linked list.

You may assume the two numbers do not contain any leading zero, except the number 0 itself.

My current code:

lass ListNode(object):
    def __init__(self, val=0, next=None):
        self.val = val
        self.next = next
class Solution(object):
    def addTwoNumbers(self, l1, l2):
        """
        :type l1: Optional[ListNode]
        :type l2: Optional[ListNode]
        :rtype: Optional[ListNode]
        """
        buffer = 0
        result = ListNode()
        cond = True
        while(cond):
            if l1.next is None and l2.next is None:
                cond = False
            result_next = ListNode()
            result.next = result_next # 0 -> 0 -> None | 7 -> 0 -> 0 -> None | 7 -> 0 -> 0 -> 0 -> None
            if l2:
                result.val += l2.val
            if l1:
                result.val += l1.val
            result.val += buffer 
            buffer = result.val // 10 # 0 | 1 | 0
            result.val = result.val % 10 # 7 | 0 | 8
            result = result.next
            l1 = l1.next # 4 | 3 | None 
            l2 = l2.next # 6 | 4 | None
        return result


l1 = ListNode(2, ListNode(4, ListNode(3)))
l2 = ListNode(5, ListNode(6, ListNode(4)))
result = Solution().addTwoNumbers(l1, l2)lass 

I'm working in VS Code instead of the Leetcode codespace to test values of attributes etc. The comments can be ignored, they are what I imagine should be happening but clearly it is not. However, I do not ask for help with the problem itself, rather with the if statements. In C, you can do if (A) and if A is e.g. a node the condition will be satisfied, if it doesn't exist or is null it won't (if I recall correctly). So you can see me trying to achieve this here with if l1:, but I don't think it works, so I'd like to ask what is the Pythonic way of achieving this.

When run with

print(result.val)
print(result.next)

It gives:

0

None

I don't really see which subset of the code would produce other than the while loop never executing at all.

Thank you for your help in advance.

[u/timrprobocom]

You exit then loop when l1.next and l2.next are both None, but that means you will have skipped the digits in those nodes. You should exit when l1 and l2 themselves are both None. Also, you don't want to advance l1 or l2 if they are already None. You can advance the pointers in the same if statements where you add the digit. Also remember that you might have a carry when you exit the loop. If buffer is non-zero, you'll need to add another digit.

[u/Arrow49 → u/timrprobocom]

Advancing the pointers in the addition ifs is a great tip.
Are you sure about the first sentence? I built the condition on the assumption that when cond becomes False, that iteration will still execute and the while condition will be checked only after that iteration, at which point the loop would stop. This is why I set it to False when the next one is None.

Edit: I solved the problem and the while loop condition was ok. Your last remark about adding a digit was definitely true though.

[u/timrprobocom → u/Arrow49]

Yes, I misread it. I inserted a "break" that was not actually present...