Leetcode (Python): Add Two Numbers

You are given two linked lists representing two non-negative numbers. The digits are stored in reverse order and each of their nodes contain a single digit. Add the two numbers and return it as a linked list.

Input: (2 -> 4 -> 3) + (5 -> 6 -> 4)
Output: 7 -> 0 -> 8

Solution:

This problem's solution is straightforward, only having to be careful to check that you have not arrived to an end of a list. We reuse the nodes to create the solution in order to use constant space.

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

class Solution:
    # @return a ListNode
    def addTwoNumbers(self, l1, l2):
        if l1 == None:
            return l2
        if l2 == None:
            return l1
        head = l1
        carryOn = (l1.val+l2.val) / 10;
        l1.val = (l1.val+l2.val) % 10;
        while l1.next != None and l2.next !=None:
            carryOn += l1.next.val
            carryOn += l2.next.val
            l1.next.val = carryOn % 10
            carryOn = carryOn // 10;
            l1 = l1.next;
            l2 = l2.next;
        if l1.next == None:
            l1.next = l2.next
        while l1.next != None:
            carryOn += l1.next.val
            l1.next.val = carryOn % 10
            carryOn = carryOn // 10;
            l1 = l1.next;
        if carryOn > 0:
            l1.next = ListNode(carryOn)
        return head