你有兩個用連結串列代表的整數，其中每個節點包含一個數字。數字儲存按照在原來整數中相反的順序，使得第一個數字位於連結串列的開頭。寫出一個函式將兩個整數相加，用連結串列形式返回和。

給出兩個連結串列 3->1->5->null 和 5->9->2->null，返回 8->0->8->null

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    /**
     * @param l1: the first list
     * @param l2: the second list
     * @return: the sum list of l1 and l2 
     */
    ListNode *addLists(ListNode *l1, ListNode *l2) {
        // write your code here
        if (l1 == NULL)
        {
            return l2;
        }else if (l2 == NULL)
        {
            return l1;
        }
        int c = 0;
        int temp = 0;
        ListNode *head = new ListNode(0);
        ListNode *p = head;
        while (l1 != NULL && l2 != NULL)
        {
            temp = l1->val+l2->val + c;
            c = temp/10;
            temp = temp%10;
            p->next = new ListNode(temp);
            p = p->next;
            l1 = l1->next;
            l2 = l2->next;
        }
        while (l1 != NULL)
        {
            temp = l1->val + c;
            c = temp/10;
            temp = temp%10;
            p->next = new ListNode(temp);
            p = p->next;
            l1 = l1->next;
        }
        while (l2 != NULL)
        {
            temp = l2->val + c;
            c = temp/10;
            temp = temp%10;
            p->next = new ListNode(temp);
            p = p->next;
            l2 = l2->next;
        }
        if (c != 0)
        {
            p->next = new ListNode(c);
        }
        return head->next;
    }
};