leetcode2. 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.

Example:
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Input: (2 -> 4 -> 3) + (5 -> 6 -> 4)
Output: 7 -> 0 -> 8
Explanation: 342 + 465 = 807.

Note:
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逆序运算，除对齐位相加，再加进位，默认进位carry=0

//cpp
class Solution(object):
    def addTwoNumbers(self, x, y):
        # T(n) = O(n)
        # S(n) = O(1)
        dummy = node = ListNode(0)
        carry = 0
        while x or y or carry:
            x_val = x.val if x else 0
            y_val = y.val if y else 0
            s = x_val + y_val + carry
            if s >= 10:
                s -= 10
                carry = 1
            else:
                carry = 0
            node.next = ListNode(s)
            x = x and x.next
            y = y and y.next
            node = node.next
        return dummy.next

# python
class Solution:
# @return a ListNode
def addTwoNumbers(self, l1, l2):
    carry = 0
    root = n = ListNode(0)
    while l1 or l2 or carry:
        v1 = v2 = 0
        if l1:
            v1 = l1.val
            l1 = l1.next
        if l2:
            v2 = l2.val
            l2 = l2.next
        carry, val = divmod(v1+v2+carry, 10)
        n.next = ListNode(val)
        n = n.next
    return root.next