## Description

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:

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


## Solutions

看清题意，这里用链表表示的数是反序的，不是从高位开始，不需要考虑数位不对等问题。再者就是，考虑为什么会出现这样的加法？难道对于大数的四则运算就是这么实现的？

### 1. 迭代

注意进位的处理！

# Time Complexity: O(n)
# Space Complexity: O(1)
# class ListNode:
#     def __init__(self, x):
#         self.val = x
#         self.next = None

class Solution:
def addTwoNumbers(self, l1: ListNode, l2: ListNode) -> ListNode:
tmp = ListNode(0)
res = tmp
carry = 0
while l1 or l2 or carry:
if l1:
carry += l1.val
l1 = l1.next
if l2:
carry += l2.val
l2 = l2.next
carry, mod = divmod(carry, 10)
tmp.next = ListNode(mod)
tmp = tmp.next
return res.next
# Runtime: 56 ms, faster than 99.76% of Python3 online submissions for Add Two Numbers.
# Memory Usage: 12.8 MB, less than 100.00% of Python3 online submissions for Add Two Numbers.