import java.util.*;

public class NQueens {
    public static boolean solve(Board solution) {
        return explore(solution, 1);
    }

    public static boolean explore(Board b, int col) {
        if (col == b.size() + 1) {
            return true;
        }
        for (int i = 1; i <= b.size(); i++) {
            if (b.safe(i, col)) {
                b.place(i, col);
                if (explore(b, col + 1)) {
                    return true;
                }
                b.remove(i, col);
            }
        }
        return false;
    }

    public static void main(String[] args) {
        giveIntro();
        Scanner console = new Scanner(System.in);
        System.out.print("What size board do you want to use? ");
        int size = console.nextInt();
        System.out.println();
        Board b = new BoardFrame(size);
        solve(b);
    }

    public static void giveIntro() {
        System.out.println("This program solves the classic '8 queens'");
        System.out.println("problem, placing queens on a chessboard so");
        System.out.println("that no two queens threaten each other.");
        System.out.println();
    }
}