Permutations.java

/*************************************************************************
 *  Compilation:  javac Permutations.java
 *  Execution:    java Permutations N
 *  
 *  Enumerates all permutations on N elements.
 *
 *  % java Permutations 3
 *  bca
 *  cba
 *  cab
 *  acb
 *  bac
 *  abc
 * 
 *  % java Permutations 3 | sort
 *  abc
 *  acb
 *  bac 
 *  bca
 *  cab
 *  cba
 *
 *************************************************************************/

public class Permutations {

    // swap the characters at indices i and j
    public static void swap(char[] a, int i, int j) {
        char c;
        c = a[i]; a[i] = a[j]; a[j] = c;
    }

    // print n! permutation of the characters of a
    public static void enumerate(char[] a, int n) {
        if (n == 1) {
            System.out.println(a);
            return;
        }
        for (int i = 0; i < n; i++) {
            swap(a, i, n-1);
            enumerate(a, n-1);
            swap(a, i, n-1);
        }
    }  

    public static void main(String[] args) {
       int N = Integer.parseInt(args[0]);
       String elements = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ";

       // initialize a[i] to ith letter, starting at 'a'
       char[] a = new char[N];
       for (int i = 0; i < N; i++)
           a[i] = elements.charAt(i);

       enumerate(a, N);
    }

}