add_two_numbers.py

# Definition for singly-linked list.

class ListNode:
    def __init__(self, x):
        self.val = x
        self.next = None

def list_to_stack(l):
    s = []
    node = l
    while node:
        s.append(node.val)
        node = node.next
    return s

def stack_to_num(s):
    num = 0
    while len(s) > 0:
        num = (num * 10) + s.pop()
    return num


class Solution:

    def addTwoNumbers(self, l1, l2):
        """
        :type l1: ListNode
        :type l2: ListNode
        :rtype: ListNode
        """
        return stack_to_num(list_to_stack(l1)) + stack_to_num(list_to_stack(l2))