leetcode[2]Add Two Numbers

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

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode *addTwoNumbers(ListNode *l1, ListNode *l2) {
      if(l1==NULL)return l2;
      if(l2==NULL)return l1;
      ListNode *l3=NULL;
      ListNode *p=l3;
      int flag=0;
      int res=0;
      while(l1!=NULL||l2!=NULL)
      {
          res=flag;
          if(l1!=NULL)
          {
              res+=l1->val;
              l1=l1->next;
          }
          if(l2!=NULL)
          {
              res+=l2->val;
              l2=l2->next;
          }
          if(l3==NULL)
          {
              l3=new ListNode(res%10);
              p=l3;
          }
          else
          {
              p->next=new ListNode(res%10);
              p=p->next;
          }
          flag=res/10;
      }
      if(flag)
      {
          p->next=new ListNode(flag);
          p=p->next;
      }
      return l3;
    }
};