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

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

class Solution(object):
    # maybe standard version
    def _addTwoNumbers(self, l1, l2):
        """
        :type l1: ListNode
        :type l2: ListNode
        :rtype: ListNode
        """
        p = dummy = ListNode(-1)
        carry = 0
        while l1 and l2:
            p.next = ListNode(l1.val + l2.val + carry)
            carry = p.next.val / 10
            p.next.val %= 10
            p = p.next
            l1 = l1.next
            l2 = l2.next
        
        res = l1 or l2
        while res:
            p.next = ListNode(res.val + carry)
            carry = p.next.val / 10
            p.next.val %= 10
            p = p.next
            res = res.next
        if carry:
            p.next = ListNode(1)
        return dummy.next
    
    # shorter version
    def addTwoNumbers(self, l1, l2):
        p = dummy = ListNode(-1)
        carry = 0
        while l1 or l2 or carry:
            val = (l1 and l1.val or 0) + (l2 and l2.val or 0) + carry
            carry = val / 10
            p.next = ListNode(val % 10)
            l1 = l1 and l1.next
            l2 = l2 and l2.next
            p = p.next
        return dummy.next