leetcode : Add Two Sum
题目：
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
思路：要考虑到进位，考虑到产生新的节点还有就是不同链的长短，都考虑进去就很好解决了。
1 class Solution(object): 2 def addTwoNumbers(self, l1, l2): 3 """ 4 :type l1: ListNode 5 :type l2: ListNode 6 :rtype: ListNode 7 """ 8 a = 0 9 head = ListNode(a) 10 q = head 11 flag = 0 12 13 while l1 and l2: 14 p = ListNode(a) 15 q.next = p 16 q = p 17 if l1.val + l2.val + flag > 10: 18 q.val = l1.val + l2.val - 10 + flag 19 flag = 1 20 elif l1.val + l2.val + flag == 10: 21 q.val = 0 22 flag = 1 23 else: 24 q.val = l1.val + l2.val + flag 25 flag = 0 26 l1 = l1.next 27 l2 = l2.next 28 29 while l1: 30 p = ListNode(a) 31 q.next = p 32 q = p 33 if l1.val + flag == 10: 34 q.val = 0 35 flag = 1 36 else: 37 q.val = l1.val + flag 38 flag = 0 39 l1 = l1.next 40 41 42 while l2: 43 p = ListNode(a) 44 q.next = p 45 q = p 46 if l2.val + flag == 10: 47 q.val = 0 48 flag = 1 49 else: 50 q.val = l2.val + flag 51 flag = 0 52 l2 = l2.next 53 54 if flag == 1: 55 p = ListNode(a) 56 q.next = p 57 q = p 58 q.val = 1 59 60 return head.next
