N Queens - all ways in Java

 

1. Missing curly braces after class declaration

java
public class Classroom 

should be:

java
public class Classroom {

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2. Wrong way of representing '.' in char

You are using:

java
board[row][j] = ".";

• This is a String, not a char.

• Since your board is char[][], use single quotes:

java
board[row][j] = '.';

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3. Strings[] args should be String[] args

java
public static void main(Strings[] args)

must be:

java
public static void main(String[] args)

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4. board.length() is wrong

length() is for methods (like String).
For arrays, use .length without parentheses:

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

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5. Your main method is misplaced

You placed main inside { } outside any method, which is invalid.
It should be inside the Classroom class, not after random { }.

---

6. Initialization of board

Same issue as point 2 — you need to use '.' instead of ".".

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✅ Here’s the corrected code:

java

import java.util.*;

public class Classroom {

    // function to place queens

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

        // base case

        if (row == board.length) {

            printBoard(board);

            return;

        }

        // column loop

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

            board[row][j] = 'Q'; // queen placed here

            nQueens(board, row + 1); // recursive call

            board[row][j] = '.'; // backtracking step

        }

    }

    // print board

    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 main(String[] args) {

        int n = 2;

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

       

        // initialize board with '.'

        for (int i = 0; i < n; i++) {

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

                board[i][j] = '.';

            }

        }

        nQueens(board, 0);

    }

}

OUTPUT: --------- chess board --------- Q . Q . --------- chess board --------- Q . . Q --------- chess board --------- . Q Q . --------- chess board --------- . Q . Q