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) {
        ListNode* root = new ListNode(0);
        add(l1, l2, root, 0);
        return root->next;
    }
   
    void add(ListNode *l1, ListNode *l2, ListNode* pre, int carry) {
        if (l1 == NULL) addSingle(l2, pre, carry);
        else if (l2 == NULL) addSingle(l1, pre, carry);
        else {
            int num = l1->val + l2->val + carry;
            pre->next = new ListNode(num % 10);
            add(l1->next, l2->next, pre->next, num/10);
        }
       
    }
   
    void addSingle(ListNode* n, ListNode* pre, int carry) {
        if (n == NULL) {
            if (carry == 0 ) return;
            else {
                pre->next = new ListNode(carry);
                return;
            }
        }
        int num = carry + n->val;
        pre->next = new ListNode(num % 10);
        addSingle(n->next, pre->next, num/10);
    }
};