2018-06-05 - 1st day - Add Two Numbers

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 contain a single digit. Add the two numbers and return it as a linked list.
You may assume the two numbers do not contain any leading zero, except the number 0 itself.

Input: (2 -> 4 -> 3) + (5 -> 6 -> 4)
Output: 7 -> 0 -> 8
Explanation: 342 + 465 = 807.

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

class Solution(object):
    def addTwoNumbers(self, l1, l2):
        """
        :type l1: ListNode
        :type l2: ListNode
        :rtype: ListNode
        """ 
        def to_int(l):
            return l.val + 10 * (to_int(l.next) if l else 0)

        num3 = to_int(l1) + to_int(l2)
        
        def to_list(val):
            l = ListNode(val % 10)
            if val > 9:
                l.next = to_list(val / 10)
            return l
            
        return to_list(num3)