// This program solves the classic "8 queens" problem using recursive 
// backtracking.

import java.util.*;

public class Queens {
	public static int counter = 0;

    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 Board(size);
        solve(b);
    }
	 
	 public static void solve(Board b) {
	 	solve(b, 0);
		System.out.println(counter);
	 }
	 
	 // pre: numQueens is the number of queens that have been
	 // safely placed on the board
	 private static boolean solve(Board b, int numQueens) {
	 	if (numQueens == 8) {
			System.out.println(b);
			counter++;
			return false;
		} else {
			for (int row = 1; row <= b.size(); row++) {
				int col = numQueens + 1;
				if (b.isSafe(row, col)) {
					b.place(row, col);
					if (solve(b, numQueens + 1)) {
						return true;
					}
					b.remove(row, col);
				}
			}
			return false;
		}
	 }

    // post: explains program to user
    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();
    }

}