Towers of Hanoi

public class Towers
{
  public static void towers (int n, char source, char spare, char dest)
  {
    if (n == 1)
      System.out.println (source + "->" + dest);
    else
    {
      towers (n - 1, source, dest, spare);
      System.out.println (source + "->" + dest);
      towers (n -1, spare, source, dest);
    }
  }

  public static void main (String[] args)
  {
    towers (3, 'A', 'B', 'C');
  }
}

Permutation

public class Permute
{
public static void printArray (char[] a)
{
  for (int i = 0; i < a.length; i++)
  {
    System.out.print (a[i] + " ");
  }
  System.out.println();
}

public static void swap (char[] a, int i, int j)
{
  char tmp = a[i];
  a[i] = a[j];
  a[j] = tmp;
}

public static void permute (char[] a, int lo)
{
  int hi = a.length;
  if (lo == hi)
  {
    printArray (a);
  }
  else
  {
    for (int i = lo; i < hi; i++)
    {
      swap (a, lo, i);
      permute (a, lo + 1);
      swap (a, i, lo);
    }
  }
}

public static void main (String[] args)
{
  char[] a = {'A', 'B', 'C', 'D'};
  permute (a, 0);
}
}

All Subsets

import java.util.*;

public class Subsets
{
  public static void printArray (int[] a)
  {
    for (int i = 0; i < a.length; i++)
    {
      if (a[i] != -1)
      {
        System.out.print (a[i] + " ");
      }
    }

    System.out.println ();
  }
  public static int[] copy (int[] a)
  {
    int[] b = new int[a.length];
    for (int i = 0; i < a.length; i++)
    {
      b[i] = a[i];
    }
    return b;
  }

  public static void subsets (int[] a, int[] b, int index)
  {
    if (index == a.length)
    {
      printArray (b);
    }
    else
    {
      int[] c = copy (b);
      b[index] = a[index];
      subsets (a, b, index + 1);
      subsets (a, c, index + 1);
    }
  }

  public static void main (String[] args)
  {
    int[] a = {15, 9, 30, 21, 19, 3, 12, 6, 25, 27};
    int[] b = new int [a.length];
    Arrays.fill (b, -1);
    subsets (a, b, 0);
  }
}

Combination

public class Combine
{
  public static void printArray (int[] b, int size)
  {
    for (int i = 0; i < size; i++)
    {
      System.out.print (b[i] + "  ");
    }
    System.out.println ();
  }

  public static void combine (int[] a, int[] b, int aIdx, int bIdx, int size)
  {
     int range = a.length - size + 1;

     if (bIdx == size)
     {
       printArray (b, size);
     }
     else
     {
       while (aIdx < range + bIdx)
       {
         b[bIdx++] = a[aIdx++];
	 combine (a, b, aIdx, bIdx, size);
	 bIdx--;
       }
     }
  }

  public static void main (String[] args)
  {
    int [] a = {1, 2, 3, 4, 5};
    int [] b = {0, 0, 0, 0, 0};
    int size = 3;
    combine (a, b, 0, 0, size);
  }
}