[LeetCode-2]Add Two Numbers(java)

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:单链表相加,大于10进1

采用递归的方法:

如果l1与l2都为空,返回进位;

如果l1为空,l2不空,返回l2的值和进位的和;

如果l1和l2都不为空,返回两者及进位的和.

/** * 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 help(l1,l2,0);    }    //递归    //都为null,返回进位    //有一个为null,返回不为null的值加进位    //都不为null,返回两者相加加进位的值    public ListNode help(ListNode l1,ListNode l2,int carry){        if(l1==null && l2==null)        return carry==0?null:new ListNode(carry);        if(l1==null && l2!=null)        l1=new ListNode(0);        if(l2==null && l1!=null)        l2=new ListNode(0);             int sum=l1.val+l2.val+carry;        ListNode l3 = new ListNode(sum%10);        l3.next=help(l1.next,l2.next,sum/10);        return l3;        }}