Here, what you need to do is divide all the fighters in two groups. So you can color them using black and red. It is obvious that fighters from same group wont fight. So fighters from red group will fight against only with fighters from black group and vice versa. Here, the list of rivals are given. So, if one is from red group then the other will be obviously from the black group. Start coloring them like this using bfs. At the end, you have two separate groups and you still don't know who are vampires and lykans. To know what is the maximum size of any group you just take maximum of red group and black group. That's it.