Check out this chapter on Hex puzzles!

Here's a pdf file you might enjoy. You may have to zoom in a lot to read the positions.

Thanks for this pdf. I can just remark that some stones are missing in Figure 8 (in m7 and n5). In this case, one can notice that there is no way to connect in the upper-right region. However, the reason of this omission could be that, by counting the number of stones, Figure 8 is an accessible position but not the original puzzle (Puzzle 3)...