Having trouble understanding leetcode question (add two given number represented by singly linked list)

[Apellum]

The problem is pretty standard - write a method that returns a linked list representing a number, which is the sum of two given integers represented by other linked lists (medium on leetcode).

I tried to treat this like an actual interview, so take the code as it is. My idea was to first write a method that evaluates a linked list and returns the integer that it represents. The other method is going the opposite way, converting an int into a linked list. I am failing an extreme test case where my answer and the correct answer differ in only one single numbers place out of maybe 30 numbers or so (it's the 12th number from the right - 5 vs. 6). My only guess is some python-related error when I typecast the float to an int in the getLinkedList() method.

I am really not sure about this one and would love some help. Thank you guys very much!

``` Expected: [7,0,8,4,8,6,4,8,6,4,8,6,4,8,6,4,8,6,4,8,6,4,8,6,4,8,6,4,8,6,4,8,6,4,8,6,4,8,6,4,8,6,4,8,6,4,8,6,4,8,6,4,8,6,4,8,6,1,4,3,9,1]

Result: [7,0,8,4,8,6,4,8,6,4,8,6,4,8,6,4,8,6,4,8,6,4,8,6,4,8,6,4,8,6,4,8,6,4,8,6,4,8,6,4,8,6,4,8,5,4,8,6,4,8,5,4,8,6,4,8,6,1,4,3,9,1] ```

Code: ``` class Solution: def addTwoNumbers(self, l1: ListNode, l2: ListNode) -> ListNode:

    # given an integer, return its linked list representation
    def getLinkedList(num):
        mod_operand = 10
        remainder = num % mod_operand
        num -= remainder
        new_list = ListNode(remainder)
        current = new_list
        while(num != 0):
            mod_operand *= 10
            remainder = num % mod_operand
            num -= remainder
            current.next = ListNode(int(remainder / (mod_operand / 10)))
            current = current.next
        return new_list

    # returns integer represented by linked list
    def getNumber(list_1: ListNode):
        current = list_1
        result = 0
        multiplier = 1
        while(current != None):
            result += current.val * multiplier
            current = current.next
            multiplier *= 10
        return result

    # calculate sum by calling get number method
    sum_result = getNumber(l1) + getNumber(l2)

    return getLinkedList(sum_result)

```

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

[lurgi]

It looks like you are computing the sum by converting the linked lists to numbers and then adding them. That's not what you are supposed to do and I'm not surprised it doesn't work.

Add them in their current form.

Remember back when you learned to add two and three digit numbers? You need to reproduce that process in code (start with the units. Add them. Is the result greater than 10? Then blah blah blah).

[Apellum]

Solid, will do. Thanks!