2. 两数相加

class Solution:
    def addTwoNumbers(self, l1, l2):
        """
        :type l1: ListNode
        :type l2: ListNode
        :rtype: ListNode
        """
        if l1 is None:
            return l2
        if l2 is None:
            return l1
        tmp = ListNode(0)
        res = tmp 
        flag = 0 
        while l1 or l2:
            tmp_sum = 0
            if l1:
                tmp_sum = l1.val 
                l1 = l1.next
            if l2:
                tmp_sum += l2.val
                l2 = l2.next 
            tmp_res = ((tmp_sum + flag) % 10) 
            flag = ((tmp_sum + flag) // 10)
            res.next = ListNode(tmp_res)
            res = res.next
            if flag:
                res.next = ListNode(1)
        res = tmp.next
        del tmp
        return res