The Eight Queen Problem

Problem

The Eight Queens Problem is a classic chess puzzle that involves placing eight queens on a chessboard in such a way that no two queens threaten each other. This means that no two queens can share the same row, column, or diagonal.

The challenge is to find all possible solutions to this problem.

Code

public class EightQueens {
    int max = 8;
    //Define an array to store the positions where the queens are placed.
    int[] array = new int[max];
    //count the number of methods
    static int count = 0;

    public static void main(String[] args) {
        EightQueens eightQueens = new EightQueens();
        eightQueens.place(0);
        System.out.println("There are " + count + " kinds of methods to solve the eight queens problem.");
    }

     /**
     * Place chess pieces in a recursive manner.
     *
     * @param n the nth queen
     */
    public void place(int n) {
        if (n == max) {
            printResult();
            count++;
            return;
        }

        for (int i = 0; i < max; i++) {
            array[n] = i;
            if (ifNotConflict(n)) {
                place(n + 1);
            }
        }
    }

    /**
     * Check whether the newly placed chess piece conflicts with the previous ones.
     * 
     * @param n the nth queen
     * @return if the queen could be placed at this place
     */
    public boolean ifNotConflict(int n) {
        for (int i = 0; i < n; i++) {
            if (array[i] == array[n] || // on the same coloum
                    Math.abs(n - i) == Math.abs(array[n] - array[i])) {  // on the same diagonal
                return false;
            }
        }
        return true;
    }

    public void printResult() {
        for (int i = 0; i < array.length; i++) {
            System.out.print(array[i] + " ");
        }
        System.out.println();
    }

}