【LeetCode】2.Add Two Numbers 链表数相加

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：采用递归思想。设置一个carray变量用于存放进位值，将l1,l2,carray相加，十进制取余得到该位的值，进行链表下一位运算。

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
public class Solution {
    public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
        return addTwoNumbersHelp(l1,l2,0);
    }
    
    public ListNode addTwoNumbersHelp(ListNode l1,ListNode l2,int carray){
        if(l1==null && l2==null){
            return carray==0?null:new ListNode(carray);
        }
        
        if(l1==null && l2!=null){
            l1=new ListNode(0);
        }
        
        if(l1!=null && l2==null){
            l2=new ListNode(0);
        }
        
        int sum=l1.val+l2.val+carray;
        ListNode curr=new ListNode(sum%10);
        curr.next=addTwoNumbersHelp(l1.next,l2.next,sum/10);
        
        return curr;
    }