## Problem
### 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
## Code
```
/**
* 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){
int flag = 0; //进位符
ListNode listNode = new ListNode(-1);//头结点,不记录数据
ListNode Nodetmp = listNode;
if(l1==null) listNode=l1;//边界条件
if(l2==null) listNode=l2;
while(l1!=null||l2!=null){//注意||
if (l1!=null){//有一个等于null后续都等于null
flag+=l1.val;
l1 = l1.next;
}
if(l2!=null){
flag+=l2.val;
l2 = l2.next;
}
Nodetmp.next = new ListNode(flag%10);
flag/=10;
Nodetmp = Nodetmp.next;
}
if(flag>0){//next都是null
Nodetmp.next = new ListNode(flag%10);
}
return listNode.next;
}
}
```