bool dfs_circle(unordered_map<int, unordered_set<int>>& edges, vector<bool>& visited, vector<bool>& on_path, int i) {
if (visited[i]) {
return false;
}
visited[i] = on_path[i] = true;
for (auto next: edges[i]) {
if (on_path[next] || dfs_circle(edges, visited, on_path, next)) {
return true;
}
}
on_path[i] = false;
return false;
}
bool canFinish(int numCourses, vector<pair<int, int>>& prerequisites) {
unordered_map<int, unordered_set<int>> edges;
for (auto p: prerequisites) {
edges[p.second].insert(p.first);
}
vector<bool> visited(numCourses, false), on_path(numCourses, false);
for (int i = 0; i < numCourses; i++) {
if (!visited[i] && dfs_circle(edges, visited, on_path, i)) {
return false;
}
}
return true;
}
bool canFinish(int numCourses, vector<pair<int, int> >& prerequisites) {
vector<vector<int> > graph(numCourses);
vector<int> vis(numCourses);
for (int i = 0; i < prerequisites.size(); i ++) {
graph[prerequisites[i].first].push_back(prerequisites[i].second);
//graph[prerequisites[i].first][prerequisites[i].second] = 1;
}
for (int i = 0; i < numCourses; i ++) {
if (vis[i] == 0) {
if (dfs_circle(graph, vis, i)) return false;
}
}
return true;
}
bool dfs_circle(vector<vector<int> >&graph, vector<int> &vis, int pos) {
vis[pos] = 1;
for (int i = 0; i < graph[pos].size(); i ++) {
int cnt = graph[pos][i];
if (cnt != pos) {
if (vis[cnt] == 1) return true;
else if (vis[cnt] == 0) {
if (dfs_circle(graph, vis, cnt)) return true;
}
}
}
vis[pos] = 2;
return false;
}
