The title, Flowers for Algernon, makes me feel that the story is most likely a tragedy. Though I have read this book before, when I consider the title alone, it makes me think of occasions when you might give someone flowers. Flowers may be given as gifts as a sign of romantic love, celebration, or as a gesture at funerals. The title mostly makes me think of a funerary offering of flowers in remembrance of someone. 

Alternately, you could guess that the story is a comedy or tragicomedy. Flowers can be a thoughtful gift to cheer someone up. Perhaps the character Algernon has been feeling sad and someone gives him flowers to help him feel better. This is a bit of a stretch, as there are no content clues in the title that Algernon has been feeling sad or requires cheering up.