Leetcode--Merge Two Sorted Lists

    static ListNode *mergeTwoLists(ListNode *l1, ListNode *l2) {
        ListNode *temp = );
        ListNode *head = temp;
        if (l1 == NULL && l2 == NULL)
            return head;
        while (l1 != NULL && l2 != NULL) {
            if (l1->val <= l2->val) {
                temp->next = new ListNode(l1->val);
                temp = temp->next;
                l1 = l1->next;
            } else {
                temp->next = new ListNode(l2->val);
                temp = temp->next;
                l2 = l2->next;
            }
        }
        if (l1 == NULL && l2 != NULL)
            temp->next = l2;
        else if (l2 == NULL && l1 != NULL)
            temp->next = l1;
        return head->next;
    }