NQueens.java

import java.util.Arrays;

public class NQueens {
	static int[] result; // this array will store the result

	// result[i]=j; means queen at i-th row is placed at j-th column.
	// Queens placed at (x1, y1) and (x2,y2)
	// x1==x2 means same rows, y1==y2 means same columns, |x2-x1|==|y2-y1| means
	// they are placed in diagonals.
	public boolean canPlace(int x2, int y2) {
		// This function will check if queen can be placed (x2,y2), or we can
		// say that Can queen at x2 row is placed at y2 column.
		// for finding the column for x2 row, we will check all the columns for
		// all the rows till x2-1.
		for (int i = 0; i < x2; i++) {
			//result[i] == y2 => columns are same
			//|i - x2| == |result[i] - y2| => in diagonals.
			if ((result[i] == y2)
					|| (Math.abs(i - x2) == Math.abs(result[i] - y2))) {
				return false;
			}
		}
		return true;
	}
	public void placeQueens(int x, int size) {
		for (int i = 0; i < size; i++) {
			//check if queen at xth row can be placed at i-th column.
			if (canPlace(x, i)) {
				result[x] = i; // place the queen at this position.
				if (x == size - 1) {
					System.out.println("Order of " + size + " queens"
							+ Arrays.toString(result));
				}
				placeQueens(x + 1, size);
			}
		}
	}
	public static void main(String[] args) {
		int n = 6;
		result = new int[n];
		NQueens i = new NQueens();
		i.placeQueens(0, n);
	}
}