leetcode merge two sorted lists

 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class  {
public:
    ListNode* mergeTwoLists(ListNode* l1, ListNode* l2) {
        if(l1 == NULL) return l2;
        if(l2 == NULL) return l1;
        ListNode *dummy = new ListNode(-1);
        ListNode *p = dummy;
        while(l1 != NULL && l2 != NULL) {
            if(l1->val > l2->val) {
                p->next = l2;
                l2 = l2->next;
            }else {
                p->next = l1;
                l1 = l1->next;
            }
            p = p->next;
        }
        if(l1 != NULL) {
            p->next = l1;
        }
        if(l2 != NULL) {
            p->next = l2;
        }
        return dummy->next;
    }
};