class Solution:
    # @param {ListNode} l1
    # @param {ListNode} l2
    # @return {ListNode}
    def addTwoNumbers(self, l1, l2):
        from itertools import izip
        def listiter(l):
            cur = l
            while cur:
                yield (False, cur.val)
                cur = cur.next
            while 1:
                yield (True, 0)

        head = ListNode(-1)
        cur = head
        carry = 0
        for (e1, x), (e2, y) in izip(listiter(l1), listiter(l2)):
            if e1 and e2:
                break
            s = x + y + carry
            s, carry = s % 10, s // 10
            node = ListNode(s)
            cur.next = node
            cur = node
        if carry > 0:
            node = ListNode(carry)
            cur.next = node

        return head.next