[LeetCode] 2. Add Two Numbers

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

class Solution:
    def addTwoNumbers(self, l1: ListNode, l2: ListNode, lso=0) -> ListNode:
        val = l1.val + l2.val + lso
        lso = val // 10
        r = ListNode(val % 10)
        
        if (l1.next != None or l2.next != None or lso != 0):
            if l1.next == None:
                l1.next = ListNode(0)
            if l2.next == None:
                l2.next = ListNode(0)
            r.next = self.addTwoNumbers(l1.next, l2.next, lso)
        
        return r