LeetCode Add Two Numbers

/**
 * 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) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        ListNode *addNum = new ListNode(0);// head-node for explicity
        ListNode *p1 = l1;
        ListNode *p2 = l2;
        ListNode *padd = addNum;
        int total;
        int flag = 0;
        
        //类似于归并
        while(p1!=NULL && p2!=NULL)
        {
            total = p1->val + p2->val + flag;
            flag = total / 10;
            total = total % 10;
            padd->next = new ListNode(total);
            padd = padd->next;
            p1 = p1->next;
            p2 = p2->next;
        }
        
        while(p1!=NULL)
        {
            total = p1->val + flag;
            flag = total / 10;
            total = total % 10;
            padd->next = new ListNode(total);
            padd = padd->next;
            p1 = p1->next;
        }
        
        while(p2!=NULL)
        {
            total = p2->val + flag;
            flag = total / 10;
            total = total % 10;
            padd->next = new ListNode(total);
            padd = padd->next;
            p2 = p2->next;
        }
        
        // 易忘最高升位！
        if(flag!=0)
        {
            padd->next = new ListNode(flag);
        }
        
        padd = addNum->next;
        delete []addNum; // free the head-node!!!
        
        return padd;
    }
};