public static int getIndexEmptySlots(Sudoku sudoku, int nSlot) { //nSlot is the n'th empty slot
		int count = 0;
		for (int index = 0; index < 9 * 9; index++) {
			if (sudoku.get(index) == 0) {
				if (count == nSlot) {
					return index;
				}
				count++;
			}
		}
		return -1;
	}