Between any pair of professional wrestlers, there may or may not be a rivalry. Suppose we have n professional wrestlers, and we have a list of r pairs of wrestlers for which there are rivalries. We want to designate each wrestler as either a good guy or as a bad guy so that every rivalry is between a good guy and a bad guy.

Give an O(n+r)-time algorithm that determines whether it is possible to designate some of the wrestlers as good guys and the remainder as bad guys so that each rivalry is between a good guy and a bad guy. If it is possible to perform such a designation, your algorithm should produce it