LeetCode 2 Add Two Number

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
2つの非負の数字を表すチェーンテーブルを2つあげます.数値は逆の順序で格納され、ノードには単一の数値が含まれます.この2つの数値を加算し、チェーンテーブルとして返します.入力:(2->4->3)+(5->6->4)出力:7->0->8
C++:

/**
 * 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){
      int sum = 0;
      ListNode *listNode = new ListNode(0);
      ListNode *p1=l1, *p2=l2, *p3=listNode;
      while(p1!=NULL||p2!=NULL)
      {
      if(p1!=NULL)
      {
          sum+=p1->val;
          p1=p1->next;
      }
      if(p2!=NULL)
      {
          sum+=p2->val;
          p2=p2->next;
      }
      p3->next=new ListNode(sum%10);
      p3=p3->next;
      sum=sum/10;
      }
      if(sum==1)
      {
     p3->next=new ListNode(1);
      }
      return listNode->next;
      }
 };