Leetcode练习－Add Two Number

You are given two linked lists representing two non-negative numbers. 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.

Input: (2 -> 4 -> 3) + (5 -> 6 -> 4)
Output: 7 -> 0 -> 8

考察链表操作的题目。 像上面的这个例子，本质是做一次加法，是342+465 ＝ 807的手算过程。 2+5=7， 4+6 ＝ 10，记录0， 进位1； 3+4 ＝ 7，加进位＝8。

实战就按照链表的结构，按顺序进行加法，同时观察进位是否为1。

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

class Solution:
# @return a ListNode
def addTwoNumbers(self, l1, l2):
if l1 == None: return l2
if l2 == None: return l1
flag = 0
dummy = ListNode(0); p = dummy
while l1 and l2:
p.next = ListNode((l1.val+l2.val+flag) % 10)
flag = (l1.val+l2.val+flag) / 10
l1 = l1.next; l2 = l2.next; p = p.next
if l2:
while l2:
p.next = ListNode((l2.val+flag) % 10)
flag = (l2.val+flag) / 10
l2 = l2.next; p = p.next
if l1:
while l1:
p.next = ListNode((l1.val+flag) % 10)
flag = (l1.val+flag) / 10
l1 = l1.next; p = p.next
if flag == 1: p.next = ListNode(1)
return dummy.next