1 struct edge{
2 int to,cost;
3 edge(){ this->to = 0; this->cost = 0;}
4 edge(int t){ this->to = t; this->cost = 0;}
5 };
6 class Solution {
7 public:
8 void addEdge(vector<edge> &edgelist, vector<vector<int>> &G, int from, int to){
9 edge e = edge(to);
10 edgelist.push_back(e);
11 G[from].push_back(edgelist.size()-1);
12 }
13 bool isCyclic(vector<edge> &edgelist, vector<vector<int>> &G, vector<bool> vis, vector<bool> recStack, int v){
14 for(int i=0;i<G[v].size();++i){
15 edge e = edgelist[G[v][i]];
16 if(recStack[e.to]) return false; // has cycle
17 if(!vis[e.to]){
18 vis[e.to] = true; recStack[e.to] = true;
19 if(!isCyclic(edgelist, G, vis, recStack, e.to)) return false; // has cycle
20 recStack[e.to] = false;
21 }
22 }
23 return true;
24 }
25 bool canFinish(int numCourses, vector<pair<int, int>>& prerequisites) {
26 vector<edge> edgelist; edgelist.clear();
27 vector<vector<int> > G(numCourses);
28 for(int i=0;i<G.size();++i) G[i].clear();
29
30 for(int i=0;i<prerequisites.size();++i){
31 pair<int, int> pr = prerequisites[i];
32 addEdge(edgelist, G, pr.first, pr.second);
33 }
34
35 vector<bool> vis(numCourses);
36 vector<bool> recStack(numCourses);
37 for(int i=0;i<numCourses;++i){
38 if(!vis[i]){
39 vis[i] = true; recStack[i] = true;
40 if(!isCyclic(edgelist, G, vis, recStack, i)) return false; // has cycle
41 recStack[i] = false;
42 }
43 }
44
45 return true;
46 }
47 };
