Find Kth from Head
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:
   *   Retrieve the kth element from this list, where
   *   the head is the 0th element.
   *
   * Time Complexity of Solution:
   *   Best = O(1); Worst = O(k).
   *
   * Technical Details: Simply count from 0 to k, meanwhile moving
   *   a pointer. Then return the pointer.
   *****************************************************************/
  public Node findKthFromHead(int k) {
    if (0 > k) {
      return null;
    }
    Node tmp = head;
    int count = 0;
    while (count < k && null != tmp) {
      tmp = tmp.next;
      count++;
    }
    return tmp;
  }
}
public class SinglyLinkedListTest {

  /**
   * Test of findKthFromHead method, of class SinglyLinkedList.
   */
  @Test
  public void testFindKthFromHead() {
    System.out.println("findKthFromHead");
    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 k = 0;
    assertEquals(input[k], linkedList.findKthFromHead(k).data);
    k = input.length / 2;
    assertEquals(input[k], linkedList.findKthFromHead(k).data);
    k = input.length - 1;
    assertEquals(input[k], linkedList.findKthFromHead(k).data);
    k = input.length + 21;
    assertEquals(null, linkedList.findKthFromHead(k));
    k = -1;
    assertEquals(null, linkedList.findKthFromHead(k));
  }
}