This question is about algorithms

Suppose you work for a lab which is studying butterflies. It has a sample of n butterflies, L1, L2, . . . , Ln. The researchers have made a series of r determinations determining whether two butterflies belong to different species. A determination is of the form (i, j), and it means that Li and Lj belong to different species. Your job is to give an O(n + r) time algorithm to decide whether the determinations are consistent with the butterflies belonging to just two species. (Note: it is possible that they could belong to three or more species, but that is a separate question.)