Method searchHelper

12_LinkedList.java/LL.java:123–136

(Node head, int key)

    }

    public int searchHelper(Node head, int key) {
        if(head == null) {
            return -1;
        }
        if(head.data == key) {
            return 0;
        }

        int idx = searchHelper(head.next, key);
        if(idx == -1) {
            return idx;
        }
        return idx+1;
    }

    public void reverse() {
        Node prev = null;