leetcode: 2. 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
#
# Definition for singly-linked list.

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

class Solution():
    def addTwoNumbers(self, l1, l2):
        dummy = Listnode(0)
        cur, carry = dummy, 0
        while l1 or l2:
            val = carry
            if l1:
                val += l1.val
                l1 = l1.next
            if l2:
                val += l2.val
                l2 = l2.next
            cur.next, carry = Listnode(val % 10), val // 10
            cur = cur.next
        if carry == 1:
            cur.next = Listnode(1)
        return dummy.next


if __name__ == '__main__':
    a, a.next, a.next.next = Listnode(2), Listnode(4), Listnode(3)
    b, b.next, b.next.next = Listnode(5), Listnode(6), Listnode(7)
    result = Solution().addTwoNumbers(a, b)
    assert '{0}->{1}->{2}->{3}'.format(result.val, result.next.val, result.next.next.val, result.next.next.next.val) \
           == '7->0->1->1'