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.

Input: (2 -> 4 -> 3) + (5 -> 6 -> 4)

Output: 7 -> 0 –> 8

本来想法（最笨的想法）是把两个链表都读出来组成数字，反转相加，再将二者的和反转存到一个链表，经谷歌提点，本来两个链表就是从个位开始的，一位一位相加，进位在下个高位继续加。

public class Solution {    public static ListNode addTwoNumbers(ListNode l1, ListNode l2) {        ListNode head = new ListNode(0);    // empty header;        ListNode resCur = head;        ListNode cursor1 = l1;        ListNode cursor2 = l2;        int remad=0;        int m=0;        while(cursor1!=null || cursor2!=null){            int sum=0;            int val1=0;            int val2=0;            if(cursor1!=null){                val1=cursor1.val;            }            if(cursor2!=null){                val2=cursor2.val;            }            sum=val1+val2+remad;            remad=sum/10;            m=sum%10;            resCur.next=new ListNode(m);            resCur=resCur.next;            if(cursor1!=null)                cursor1=cursor1.next;            if(cursor2!=null)                cursor2=cursor2.next;        }        if(remad!=0)            resCur.next=new ListNode(remad);        return head.next;    }    public static  void main(String[] args){        ListNode l1=new ListNode(5);        l1.next=new ListNode(4);        ListNode l2=new ListNode(5);        l2.next=new ListNode(5);        ListNode resNum=new ListNode(0);        resNum=addTwoNumbers(l1,l2);        while (resNum!=null){            System.out.print(resNum.val+"->");            resNum=resNum.next;        }    }}