leetcode 207. Course Schedule

 1 vector<unordered_set<int>> makegraph(int numCourses, vector<pair<int, int>> prerequisites){
 2     vector<unordered_set<int>> graph(numCourses);
 3    for(auto pre:prerequisites) graph[pre.second].insert(pre.first);
 4    return graph;
 5 }
 6 vector<int> compute_degrees(vector<unordered_set<int>> graph){
 7    vector<int> degrees(graph.size(),0);
 8     for(auto n:graph)
 9        for(int neigh:n)
10           degrees[neigh]++;
11    return degrees;
12 }
13 bool canFinish(int numCourses, vector<pair<int, int>>& prerequisites) {
14    vector<unordered_set<int>> graph=makegraph(numCourses,prerequisites);
15    vector<int> degrees=compute_degrees(graph);
16    for(int i=0;i<numCourses;i++){
17      int j=0;
18      for(j;j<numCourses;j++)
19         if(!degrees[j]) break;
20      if(j==numCourses) return false;
21      degrees[j]=-1;
22      for(int neigh:graph[j])
23          degrees[neigh]--;
24     }
25    return true;
26   }
27 };

 1 //dfs
 2 vector<unordered_set<int>> makegraph(int numCourses, vector<pair<int, int>>& prerequisites){
 3    vector<unordered_set<int>> graph(numCourses);
 4    for(auto pre:prerequisites)
 5       graph[pre.second].insert(pre.first);
 6    return graph;
 7 }
 8 bool dfscycle(vector<unordered_set<int>>& graph,int i,vector<bool>& onpath,vector<bool>& visit){
 9    if(visit[i]) return false;
10    visit[i]=onpath[i]=true;
11    for(int neigh:graph[i])
12        if(onpath[neigh]||dfscycle(graph,neigh,onpath,visit))
13           return true;
14     return onpath[i]=false;
15 }
16 bool canFinish(int numCourses, vector<pair<int, int>>& prerequisites) {
17     vector<unordered_set<int>> graph=makegraph(numCourses,prerequisites);
18     vector<bool> visit(numCourses,false),onpath(numCourses,false);
19     for(int i=0;i<numCourses;i++)
20        if(!visit[i]&&dfscycle(graph,i,onpath,visit))
21           return false;
22     return true;
23 }