Count the number of words over an alphabet of n letters without non-overlapping repeated strings of length 2

Question132847.java

import java.util.Set;
import java.util.HashSet;

public class Question132847 {
    static int n;
    static int count;

    static Set<String> pairs = new HashSet<String> ();

    public static void main (String [] args) {
        for (n = 1;n < 5;n++) {
            count = 0;
            recurse (' ',"");
            System.out.println (n + " : " + count);
        }
    }

    static void recurse (char last,String nextPair) {
        count++;
        for (int k = 0;k < n;k++) {
            char next = (char) ('a' + k);
            String pair = last + "" + next;
            if (!pairs.contains (pair)) {
                boolean added = pairs.add (nextPair);
                recurse (next,pair);
                if (added)
                    pairs.remove (nextPair);
            }
        }
    }
}