Place N amount of queens in such a way that none of them can attack each other.

import java.util.*;

public class NQueensRewrite {
    public static void main(String[] args) {
        Scanner input = new Scanner(System.in);
        System.out.println("Enter amount of queens");
        int n = input.nextInt();
        int[] board = new int[n];

    public static void fillBoard(int[] board) {
        int queens = 1;
        while (queens < board.length) {
            boolean validColumnFound = false;
            //Start from previous confirmed column or check all if valid column found in row
            for (int column = board[queens] + 1; column < board.length; column++) {
                if (isValid(column, queens, board)) {
                    board[queens] = column;
                    validColumnFound = true;
            //If no other valid column is found, then clear the row and go back to try another column
            if (!validColumnFound) {
                board[queens] = 0;

    public static boolean isValid(int column, int row, int[] board) {
        //Check if any other row has the specified column
        for (int i = 0; i < row; i++) {
            if (board[i] == column) {
                return false;

            //Check the differences bewteen the spaces if they are uniform (even)
            //If the difference between the column of ith row and column being checked is
            //the same as the difference between the current row and row being checked,
            //then some previous queen is diagnal to the current spot being checked
            if (Math.abs(board[i] - column) == Math.abs(i - row)) {
                return false;
        return true;

    public static void printBoard(int[] board) {
        for (int i = 0; i < board.length; i++) {
            for (int j = 0; j < board.length; j++) {
                if (board[i] == j) {
                else {
                    System.out.print("| ");
  • 2
    \$\begingroup\$ I've given answers to N-Queens questions before. A Java one and a C++ one. I don't know how well they would apply to your code, but they might be worth reading. There are surely other good answers on the site, too. \$\endgroup\$
    – Veedrac
    May 11, 2015 at 14:25
  • \$\begingroup\$ Follow-up question \$\endgroup\$ Jun 1, 2015 at 22:27

1 Answer 1


It took me a while to figure out your code. I have solved this problem a couple of times before, but found your code hard to read. In large part, because your variable names are misleading....

  • board is not a board, it is an array of int, with the value of the queen's column. I see the board as being a 2D matrix, you have it as a 1D array.
  • queens is not the number of queens, but the number of rows you have currently populated.

Then, your code is not as general-purpose as I was expecting.... Your code only ever finds solutions where the position (0,0) is a queen. Now, that's not to say that solutions with a queen there are wrong, but, are you sure? What if there is no solution with a queen in the corner....

If you know for sure there will be one, then you should at least document that..... if not, then it's a bug.

All in all, your code could use some more help to get it readable.

  • \$\begingroup\$ Ah I think I understand... What are some good resources or guidelines I could use to begin to write more readable code? \$\endgroup\$
    – FabZeros
    May 12, 2015 at 14:53

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