private boolean isValidCell(ArrayList<Integer> cell) {
		if (cell.size() != numDimension ) return false;
		for (int i=0; i<numDimension; i++ ) {
			// coordinate start at 0
			if ( cell.get(i) < 0 ) return false;
			// coordinate end at dimSize[i] -1 
			if (cell.get(i) >= dimSize[i]) return false;
		}
		return true;
	}