Reverse in Pair
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).
   *
   * Description: Reverse each pair of nodes, such that abcdefg
   *   becomes badcfeg.
   *****************************************************************/
  public void reverseInPairs() {
    for (Node t = head; null != t && null != t.next; t = t.next.next) {
      swap(t, t.next);
    }
  }

  private void swap(Node a, Node b) {
    int tmp = a.data;
    a.data = b.data;
    b.data = tmp;
  }
}
public class SinglyLinkedListTest {

  /**
   * Test of reverseInPairs method, of class SinglyLinkedList.
   */
  @Test
  public void testReverseInPairs() {
    System.out.println("reverseInPairs");
    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()));
    linkedList.reverseInPairs();
    int[] exp = {4, 9, 2, 5, 12, 1, 7, 6, 8, 4, 0, 3, 19, 16, 11};
    assertTrue(Arrays.equals(exp, linkedList.toArray()));
  }
}