NQueens.java

import java.io.*;
public class NQueens {

    static int queens(boolean[][] board, int row){
        if(row == board.length){
            display(board);
            System.out.println();
            return 1;
        }

        int count =0;

        for(int col = 0; col< board.length ; col++){
            if(isSafe(board, row, col)){
                board[row][col]=true;
                count = count + queens(board, row+1);
                board[row][col] = false;
            }            
        }
       
        return count;

    }


    static boolean isSafe(boolean[][] board, int row, int col){
        for(int i = 0; i  <row ;i++){
            if(board[i][col])
            return false;
        }

        int minLeft = Math.min(row, col);
        for(int i = 1; i<=minLeft; i++){
            if(board[row-i][col-i]){
                return false;
            }
        }

        int minRight = Math.min(row, board.length - col -1);
        for(int i = 1; i<=minRight; i++){
            if(board[row-i][col+i]){
                return false;
            }
        }
      return true; 
    }

    static void display(boolean[][] board){
        for(boolean[] row: board){
            for(boolean element: row){
                if(element){
                    System.out.print("Q ");
                }
                else{
                    System.out.print("X ");
                }                
            }
            System.out.println();
        }
    }
    
    public static void main(String[] args){
    boolean[][] board = new boolean[4][4];
    System.out.println(queens(board  ,0));
    }
}