Vector<Integer> getCoil() {
        int numCells = countCells();
        Vector<Integer> result = new Vector<>(numCells);
        Util.initialize(result, -2);

        int i = 0;
        int j = 0;
        int ring = 0;

        while (i < numCells) {
            int index = tryToGetIndex(ring, j);
            if (index >= 0) {
                result.set(i, index);
                i++;
            }
            j = j + 1;
            if (j >= 6 * ring) {
                ring = ring + 1;
                j = 0;
            }
            if (ring > numRows + numColumns) {
                return result;
            }
        }

        return result;
    }