Dumb Eight Queen Solution in Java

Board.java

/* Board.java
 *
 * Part of the Eight Queen puzzle solver by Asad ur Rehman <asad.rehman@gmail.com>
 *
 */

public class Board {
	private int n;
	private byte[][] values;
	private int num_queens;

	public Board(int dim) {
		n = dim;
		values = new byte[n][n];		// automatically initialized to 0x0
		num_queens = 0;
	}

	public Board(Board b) {
		n = b.n();
		values = new byte[n][n];
		for(int i = 0; i < n; i++)
			for(int j = 0; j < n; j++)
				values[i][j] = b.values[i][j];		

		num_queens = b.num_queens();
	}
	
	public void markQueen(int r, int c) {		
		for(int i = 0; i < n; i++) {
			for(int j = 0; j < n; j++) {
				if(i == r && j == c)
					values[i][j] = 'Q';
				else if( (i == r) || (j == c) || (r-c) == (i-j) || (r+c) == (i+j) )
					values[i][j] = 'X';
			}
		}
		num_queens++;
	}
	
	public String asString() {
		StringBuilder sb = new StringBuilder();
		
		for(int i = 0; i < n(); i++)
			for(int j = 0; j < n(); j++)
				sb.append(values[i][j]);
				
		return sb.toString();
	}

	public boolean isEmptyAt(int r, int c) {
		return (values[r][c] == 0x0);
	}

	public int n() {
		return n;
	}

	public byte[][] values() {
		return values;
	}

	public int num_queens() {
		return num_queens;
	}
}

Solution.java

/*
 * Solution.java
 *
 * 8-Queen puzzle solver by Asad ur Rehman <asad.rehman@gmail.com> 
 *  
 * Sun Mar 11 01:14:39 PKT 2012
 *
 * real	0m1.644s
 * user	0m2.385s
 * sys	0m0.126s
 * 
 */

import java.util.HashSet;

public class Solution {

	private HashSet<String> cache;
	
	public static void main(String[] args) {
		Solution s = new Solution();
		Board b = new Board(8);
		s.solve(b);
	}
	
	public Solution() {
		cache = new HashSet<String>();
	}
	
	void solve(Board b) {
		if(cache.contains(b.asString()))
			return;
		else
			cache.add(b.asString());
		
		if(b.n() == b.num_queens()) {
			System.out.println("Found " + b.num_queens() + " queens solution");
			return;
		}

		for(int i = 0; i < b.n(); i++) {
			for(int j = 0; j < b.n(); j++) {
				if(b.isEmptyAt(i,j)) {
					Board tmp = new Board(b);
					tmp.markQueen(i,j);
					solve(tmp);
				}
			}
		}
		
	}

}