public void addLast(int value) {
  Node last = findLast();
  last.next = new Node(value, null);
}
public void addFirst(int value) {
  Node newNode = new Node(value);
  newNode.next = this.head.next;
  this.head.next = newNode;
}

public void insert(int index, int value) {
  Node prev = findNode(index - 1);  
  if (prev != null) {
    Node newNode = new Node(value);
    newNode.next = prev.next;
    prev.next = newNode;
  }
}
public void remove(int index) {
  Node prev = findNode(index - 1); Node curr;
  if (prev != null && (curr = prev.next) != null) {
      prev.next = curr.next;
  }
}