Counting Sort
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;

  /************************************************************************
   * Time Complexity of Solution:
   *   O(n*log n).
   *
   * Description: sort this LinkedList
   *
   * Technical Details: If the elements of this LinkedList fall within a
   *   known short range, then it makes sense to use an integer algorithm
   *   like counting sort (ref geekviewpoint/java/sorting/counting_sort),
   *   since integer algorithms are very fast.
   *
   *   To keep the implementation simple, assume the elements range from 0
   *   to max, inclusive. Counting sort then proceeds by creating a bucket
   *   for each key; incrementing a counter each time a key recurs in the
   *   list; then emptying the buckets back into the LinkedList.
   ************************************************************************/
  public void countingSort(int max) {
    //create a bucket for each key
    int A[] = new int[max + 1];//Java initializes int arrays to 0.
    //count recurrence of keys
    for (Node t = head; null != t; t = t.next) {
      A[t.data]++;
    }
    //swap sorted data back into LinkedList
    Node t = head;
    for (int i = 0; i < A.length; i++) {
      for (int x = 0; x < A[i]; x++) {
        t.data = i;
        t = t.next;
      }
    }

  }
}
public class SinglyLinkedListTest {

   /**
   * Test of countingSort method, of class SinglyLinkedList.
   */
  @Test
  public void testCountingSort() {
    System.out.println("countingSort");
    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]);
    }
    assertTrue(Arrays.equals(input, linkedList.toArray()));
    Arrays.sort(input);
    assertFalse(Arrays.equals(input, linkedList.toArray()));
    linkedList.countingSort(19);
    assertTrue(Arrays.equals(input, linkedList.toArray()));
  }
}