leetcode@ [207] Course Schedule

struct edge{
    int to,cost;
    edge(){ this->to = 0; this->cost = 0;}
    edge(int t){ this->to = t; this->cost = 0;}
};
class Solution {
public:
    void addEdge(vector<edge> &edgelist, vector<vector<int>> &G, int from, int to){
        edge e = edge(to);
        edgelist.push_back(e);
        G[from].push_back(edgelist.size()-1);
    }
    bool isCyclic(vector<edge> &edgelist, vector<vector<int>> &G, vector<bool> vis, vector<bool> recStack, int v){
        for(int i=0;i<G[v].size();++i){
            edge e = edgelist[G[v][i]];
            if(recStack[e.to]) return false; // has cycle
            if(!vis[e.to]){
                vis[e.to] = true; recStack[e.to] = true;
                if(!isCyclic(edgelist, G, vis, recStack, e.to)) return false; // has cycle
                recStack[e.to] = false;
            }
        }
        return true;
    }
    bool canFinish(int numCourses, vector<pair<int, int>>& prerequisites) {
        vector<edge> edgelist; edgelist.clear();
        vector<vector<int> > G(numCourses);
        for(int i=0;i<G.size();++i) G[i].clear();
        
        for(int i=0;i<prerequisites.size();++i){
            pair<int, int> pr = prerequisites[i];
            addEdge(edgelist, G, pr.first, pr.second);
        } 
        
        vector<bool> vis(numCourses);
        vector<bool> recStack(numCourses);
        for(int i=0;i<numCourses;++i){
            if(!vis[i]){
                vis[i] = true; recStack[i] = true;
                if(!isCyclic(edgelist, G, vis, recStack, i)) return false; // has cycle
                recStack[i] = false;
            }
        }
        
        return true;
    }
};