题目： Add Two Numbers 

题目描述： 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.

程序设计

链表节点定义 

Definition for singly-linked list. 

class ListNode(object): 

def init(self, x): 

self.val = x 

self.next = None

函数体 

class Solution(object): 

def addTwoNumbers(self, l1, l2): 

return self.add(l1, l2) 

def add(self, l1, l2, s=0): #递归函数 

l = ListNode(s) 

if l1: 

l.val += l1.val 

l1 = l1.next 

if l2: 

l.val += l2.val 

l2 = l2.next 

s = l.val/10 

l.val = l.val % 10 

if l1 or l2 or s: l.next = self.add(l1, l2, s) #递归边界 

return l