Add Two Numbers

## 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;
    }
}
```