[LeetCode] 2. Add Two Numbers 两数相加 @python

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.

给定两个非空链表来代表两个非负数，位数按照逆序方式存储，它们的每个节点只存储单个数字。将这两数相加会返回一个新的链表。

你可以假设除了数字 0 之外，这两个数字都不会以零开头。

示例：

输入：(2 -> 4 -> 3) + (5 -> 6 -> 4)
输出：7 -> 0 -> 8
原因：342 + 465 = 807

Solution

添加一个flag，然后链表诸位相加为tmpsum，若大于9则flag为1，下一位相加时加上flag。注意最后一位，当最后flag为1时，要在结果的表尾加上1。

# -*- coding: utf-8 -*-
"""
Created on Sat Mar 17 15:34:35 2018

@author: Saul
"""

# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, x):
#         self.val = x
#         self.next = None

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:
            tmpsum = 0
            if l1:
                tmpsum = l1.val
                l1 = l1.next
            if l2:
                tmpsum += l2.val
                l2 = l2.next
            tmpres = ((tmpsum + flag) % 10)
            flag = ((tmpsum + flag) // 10)
            res.next = ListNode(tmpres)
            res = res.next
        if flag:
            res.next = ListNode(1)
        res = tmp.next
        del tmp
        return res