# 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
        """
        l1 = self.reverseList(l1)
        l2 = self.reverseList(l2)
        head1 = l1
        head2 = l2
        count = 0
        while head1 and head2:
            sum = head1.val + head2.val + count
            head1.val = sum % 10
            count = sum / 10
            if count and not head1.next and not head2.next:
                head1.next = ListNode(count)
                count = 0
            if not head1.next and head2.next: head1.next = ListNode(0)
            if not head2.next and head1.next: head2.next = ListNode(0)
            head1 = head1.next
            head2 = head2.next
        return self.reverseList(l1)
    def reverseList(self, head):
        """
        :type head: ListNode
        :rtype: ListNode
        """
        if not head: return head
        if not head.next: return head
        reversedList = self.reverseList(head.next)
        head.next.next = head
        head.next = None
        return reversedList