Leetcode#21 Merge two sorted lists

class Solution {
    public:
        ListNode* mergeTwoLists(ListNode* l1, ListNode* l2) {
            ListNode* head = NULL;
            if(l1 == NULL)
                return l2;
            if(l2 == NULL)
                return l1;

            if(l1->val <= l2->val) {
                head = l1;
                l1 = l1->next;
            } else {
                head = l2;
                l2 = l2->next;
            }

            ListNode* dummy_head = head;

            while (l1 != NULL && l2 != NULL) {
                if(l1->val <= l2->val) {
                    head->next = l1;
                    head = head->next;
                    l1 = l1->next;
                }else{
                    head->next = l2;
                    head = head->next;
                    l2 = l2->next;
                }
            }

            if (l1 != NULL) {
                head->next = l1;
            } else if (l2 != NULL) {
                head->next =l2;
            } else {

            }

            return dummy_head;
        }
};