leetcode 2 Add Two Numbers

第一次写的有些麻烦，不好，问题关键是默认两个链表长度相等，前面补0即可。

/**
 * 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) {
         if(l1 == null) return l2;
         if(l2 == null) return l1;
         
         ListNode result = new ListNode(0);
         ListNode res = new ListNode(0);
         result = res;
         int flag = 0;
         
         while(l1 != null || l2 != null)
         {
             int a1 = l1==null?0:l1.val;
             int a2 = l2==null?0:l2.val;
             int a = a1+a2+flag;
             flag = a / 10;
             ListNode t = new ListNode(a%10);
             res.next = t;
             res = t;
             l1 = l1==null?null:l1.next;
             l2 = l2==null?null:l2.next;
         }
         
         if(flag == 1)
         {
             ListNode t = new ListNode(1);
             res.next = t;
             res = t;
         }
         
         return result.next;
         
        
    }
}