bool hasCycle(int course, vector<vector<int> >& adj, vector<bool>& visited) {

visited[course] = true;

for(int i = 0; i < (int)adj[course].size(); ++i) {

if(visited[adj[course][i]]) {

return true;

}

if(hasCycle(adj[course][i], adj, visited)) {

return true;

}

}

visited[course] = false;

return false;

}

enum Color {

WHITE, GREY, BLACK

};

bool hasCycle2(int course, vector<vector<int>> const& adj, vector<int>& color) {

color[course] = GREY;

for(int i = 0; i < (int)adj[course].size(); ++i) {

int neigh = adj[course][i];

if(color[neigh] == GREY) {

return true;

}

if(color[neigh] == WHITE) {

if(hasCycle2(neigh, adj, color)) {

return true;

}

}

}

color[course] = BLACK;

return false;

}

bool canFinish(int numCourses, vector<pair<int, int>>& prerequisites) {

if(numCourses < 1) return true;

vector<vector<int> > adj;

adj.resize(numCourses);

int edges = prerequisites.size();

for(int i = 0; i < edges; ++i) {

int u = prerequisites[i].first;

int v = prerequisites[i].second;

adj[u].push_back(v);

}

vector<bool> visited(numCourses, false);

vector<int> color(numCourses, WHITE);

/*

for(int course = 0; course < numCourses; ++course) {

if(color[course] == WHITE) {

if(hasCycle2(course, adj, color))

return false;

}

}

return true;

}