Add Before
by Isai Damier

/***************************************************************************
 * Author: Isai Damier
 * Title: Singly Linked List
 * Project: geekviewpoint
 * Package: datastructure
 *
 * Description: A LinkedList is a data structure that allows access
 *   to a collection of data using pointers/references. While an
 *   array can also be defined as above, LinkedLists and arrays differ
 *   in how they are stored in memory and in the operations they
 *   allow. Unlike an array that must be stored in a block of memory,
 *   the nodes of a LinkedList can be stored anywhere because each
 *   node has a reference to the node that succeeds it. Because the
 *   nodes are stored so loosely, inserting nodes into a LinkedList
 *   is easy; whereas in an array, all the succeeding elements must
 *   be shifted. Of course, insertion also means changing the size of
 *   the array, which means creating the entire array anew.
 *
 *   Perhaps the greatest beauty of LinkedList is that it allows
 *   accessing an entire sequence of nodes using only one variable:
 *   a reference to the first node in the sequence.
 *
 *   Countless operations can be performed on LinkedLists. Following
 *   are a few, ranging from the common to the very interesting.
 **************************************************************************/ 
 public class SinglyLinkedList {

  Node head = null;
  Node tail = null;

  /*****************************************************************
   * Statement:
   *   Add element el before node n
   *
   * Time Complexity of Solution:
   *   Best = O(1); Worst = O(n).
   *
   * Technical Details: The idea here is to insert element el between node
   *   n and it's parent. So if node is the head of the list, el is added
   *   before node n and thus becomes the new head; else, we traverse the
   *   list looking for node n and it's parent so to add el between n
   *   and the parent. If node n is not on the list, then element el
   *   is added as the tail of the list.
   *****************************************************************/
  public void addBefore(int el, Node n) {
    if (null == head || n.data == head.data) {
      addToHead(el);
    } else {
      Node parent = null;
      Node curr = head;
      while (null != curr && curr.data != n.data) {
        parent = curr;
        curr = curr.next;
      }
      if (null != curr) {
        parent.next = new Node(el, n);
      } else {
        addToTail(el);
      }
    }
  }
}
public class SinglyLinkedListTest {

   /**
   * Test of addBefore method, of class SinglyLinkedList.
   */
  @Test
  public void testAddBefore() {
    System.out.println("addBefore");
    int[] input = {9, 4, 5, 2, 1, 12, 6, 7, 4, 8, 3, 0, 16, 19, 11};
    SinglyLinkedList linkedList = new SinglyLinkedList();
    for (int i = 0; i < input.length; i++) {
      linkedList.addToTail(input[i]);
    }
    int value = 51;
    linkedList.addBefore(value, linkedList.find(7));
    int[] expected = {9, 4, 5, 2, 1, 12, 6, 51, 7, 4, 8, 3, 0, 16, 19, 11};
    assertTrue(Arrays.equals(expected, linkedList.toArray()));
  }
}