NQueen2.java

public class NQueen2 {
    public static boolean isSafe(char board[][], int row, int col) {
        // vertically up
        for (int i = row - 1; i >= 0; i--) {
            if (board[i][col] == 'Q') {
                return false;
            }
        }
        // diagonally left up
        for (int i = row - 1, j = col - 1; i >= 0 && j >= 0; i--, j--) {
            if (board[i][j] == 'Q') {
                return false;
            }
        }
        // diagonally right up
        for (int i = row - 1, j = col + 1; i >= 0 && j < board.length; i--, j++) {
            if(board[i][j] == 'Q'){
                return false;
            }
        }
        return true;
    }
    public static int count = 0;

    public static void nQueen(char board[][], int row) {
        if (row == board.length) {
            count++;
            return;
        }
        for (int j = 0; j < board.length; j++) {
            if (isSafe(board, row, j)) {
                board[row][j] = 'Q';
                nQueen(board, row + 1);
                board[row][j] = 'x';
            }
        }
    }

    public static void main(String args[]) {
        int n = 15;
        char board[][] = new char[n][n];
        for (int i = 0; i < board.length; i++) {
            for (int j = 0; j < board.length; j++) {
                board[i][j] = 'x';
            }
        }
        nQueen(board, 0);
        System.out.println(count);
    }
}