leetcode--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) {
        ListNode *p = l1;
        ListNode *q = l1;
        int tem = 0;
        while(l1 != NULL && l2 != NULL)
        {
            l1->val = l2->val + tem + l1->val;
            if(l1->val >= 10)
            {
                tem = l1->val / 10;
                l1->val %= 10;
            }else
            {
                tem = 0;
            }
            q = l1;
            l1 = l1->next;
            l2 = l2->next;
        }
        if(l2 != NULL)
        {
            q->next = l2;
            l1 = l2;
        }
        while(l1 != NULL)
        {
            l1->val = tem + l1->val;
            if(l1->val >= 10)
            {
                tem  = l1->val/10;
                l1->val %= 10;
            }else
            {
                tem = 0;
            }
            q = l1;
            l1 = l1->next;
        }
        if(tem != 0)
        {
            ListNode *n = new ListNode(1);
            q->next = n;
            n->val = tem;
        }
        return p;
    }
};

好多细节没注意到就出bug，写程序需要先把思路想好再做。