Use recursion to implement combination of a set of characters

Combination.java

public class Combination
{
    public static void combinate(String prefix, String s)
    {
	StdOut.println(prefix);
	int N = s.length();
	if (N == 0) return;
	
	for (int i = 0; i < N; i++)
	    combinate(prefix + s.charAt(i), s.substring(i+1, N));
	
    }
    public static void combinateK(String prefix, String s, int k)
    {
	int N = prefix.length();
	int M = s.length();
	if (N == k)
	    {
		StdOut.println(prefix);
		return ;
	    }
	for(int i = 0; i < M; i++)
	    {
		combinateK(prefix + s.charAt(i), s.substring(0, i), k);
	    }
    }
    public static void main(String[] args)
    {
	String alphabet = "abcdefghijklmnopqrstuvwxyz";
	int N = Integer.parseInt(args[0]);
	int K = Integer.parseInt(args[1]);
	String elements = alphabet.substring(0, N);
	combinateK("", elements, K);
    }
}