The police department in the city of Computopia has made all streets one-way. The mayor con- tends that there is still a way to drive legally from any intersection in the city to any other intersection, but the opposition is not convinced. A computer program is needed to determine whether the mayor is right. However, the city elections are coming up soon, and there is just enough time to run a linear-time algorithm.

Formulate the problem graph theoritically, and explain why it can indeed be solved in linear time.