【leetcode】Add Two Numbers

题目：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.

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

class ListNode {      int val;      ListNode next;      ListNode(int x) { val = x; } }public class Solution {    public ListNode addTwoNumbers(ListNode l1, ListNode l2) {        return addTwo(l1,l2,0);    }    public ListNode addTwo(ListNode l1,ListNode l2,int s){        if(l1==null && l2==null){            return s==0?null:new ListNode(s);        }        if(l1==null&&l2!=null){            l1=new ListNode(0);        }        if(l2==null&&l1!=null){            l2 = new ListNode(0);        }        int sum = l1.val+l2.val+s;        ListNode curr = new ListNode(sum%10);        curr.next = addTwo(l1.next,l2.next,sum/10);        return curr;    }    public static void main(String[] args){    }}