public static void main(String args[]){

int n=4;

char board[][] = new char[n][n];

int row=0;

// initialize

for(int i=0; i<n; i++){

for(int j=0; j<n; j++){

board[i][j] = '.';

}

}

nQueen(board, row);

System.out.println("Total number of boards for n queens: " + count);

}

static int count = 0;

public static void printBoard(char board[][]){

System.out.println("----------chess board--------------");

for(int i = 0; i<board.length; i++){

for(int j=0; j<board.length; j++){

System.out.print(board[i][j] + " ");

}

System.out.println();

}

}

public static void nQueen(char board[][], int row){

// base

if(row == board.length){

printBoard(board);

count++;

return;

}

// column loop

for(int j = 0; j<board.length; j++){

if(isSafe(board, row, j)){

board[row][j] = 'Q';

nQueen(board, row+1); // function call

board[row][j] = '.'; // backtracking step (removing the queen bec each row can have only one queen)

}

}

}

public static boolean isSafe(char board[][], int row, int col){

// 3 conditions to check for attacks

// 1. ek col me ek hi queen (Vertical up Check)

for(int i=row-1; i>=0; i--){

if(board[i][col] == 'Q'){

return false;

}

}

// 2. no queens in diagonal left up (Diagonal up Check)

for(int i=row-1, j = col-1; i>=0 && j>=0; i--, j--){

if(board[i][j] == 'Q'){

return false;

}

}

// 3. no queens in diagonal right up (Diagonal up Check)

for(int i=row-1, j = col+1; i>=0 && j<board.length; i--, j++){

if(board[i][j] == 'Q'){

return false;

}

}

return true;

}

}