solve

public class HexagonCell {
    boolean state;
    Set<Integer> nbs;

    HexagonCell(boolean state, Set<Integer> nbs) {
        this.state = state;
        this.nbs = nbs;
    }
}

public int get_index(int x, int y) {
    int index = 0;
    for (Hexagon hex1 : hexagons) {
        if (hex1.coordX == x && hex1.coordY == y) {
            return index;
        }
        index++;
    }
    return -1;
}

public HexagonCell[] prepare_cells() {
    HexagonCell[] out = new HexagonCell[hexagons.size()];

    int[][] nbs1 = nbs;

    boolean isOdd = type.equals("odd");

    int index = 0;
    for (Hexagon hex1 : hexagons) {
        Set<Integer> eq = new HashSet<>();
        eq.add(index);

        int x = hex1.coordX;
        int y = hex1.coordY;

        if (isOdd) {
            nbs1 = y % 2 == 0 ? nbsOdd : nbsEven;
        }

        for (int[] nb : nbs1) {
            int idx = get_index(x + nb[0], y + nb[1]);

            if (idx != -1) {
                eq.add(idx);
            }
        }
        out[index++] = new HexagonCell(hex1.state, eq);
    }

    return out;
}

public Set<Integer> unique_set(Set<Integer> a, Set<Integer> b) {
    Set<Integer> out = new HashSet<>();

    for (int i : a) {
        if (!b.contains(i)) {
            out.add(i);
        }
    }

    for (int i : b) {
        if (!a.contains(i)) {
            out.add(i);
        }
    }

    return out;
}

public HexagonCell[] solve() {
    HexagonCell[] cells = prepare_cells();

    int len = cells.length;

    for (int i = 0; i < len; i++) {
        for (int j = i; j < len; j++) {
            if (cells[j].nbs.contains(i)) {
                HexagonCell t = cells[i];
                cells[i] = cells[j];
                cells[j] = t;
                break;
            }
        }

        for (int j = i + 1; j < len; j++) {
            if (cells[j].nbs.contains(i)) {
                cells[j].state ^= cells[i].state;
                cells[j].nbs = unique_set(cells[j].nbs, cells[i].nbs);
            }
        }
    }

    for (int i = len - 1; i >= 0; i--) {
        for (int j = 0; j < i; j++) {
            if (cells[j].nbs.contains(i)) {
                cells[j].state ^= cells[i].state;
                cells[j].nbs = unique_set(cells[j].nbs, cells[i].nbs);
            }
        }
    }
    return cells;
}