207. curriculum - graph theory, depth traversal

class Solution {
    
public:
    bool flag = true;
    vector<int> visited;
    vector<vector<int>> vecVecInt;
    bool canFinish(int numCourses, vector<vector<int>>& prerequisites) {
    
        // It is actually a graph theory problem , See if there is a ring 
        // So we just look at each course as a point ,  Prerequisite course relationship as a line , Then observe whether there is a ring 
        visited.resize(numCourses); // structure numCourses The size of visited Array , Record whether the point has been visited 
        vecVecInt.resize(numCourses);   // structure numCourses The size of vecVecInt Array , Record the connection between this point and other points 
        for(auto i : prerequisites){
        // Traverse all prerequisite relationships , Structure 
            vecVecInt[i[0]].push_back(i[1]);
        }
        for(auto i = 0; i < numCourses && flag; ++i){
       // Traverse each point , Observe whether there is a ring from this point 
            dfs(i);
        }
        return flag;    //flag Used to record whether a ring exists in the global , The default is true, Existence as false
    }
    void dfs(int n){
        // Depth traversal graph 
        if(visited[n] == 2) //2 Indicates that the point has been visited , And not the point on the ring 
            return;
        visited[n] = 1; // Mark as the point visited 
        for(auto i : vecVecInt[n]){
    
            if(visited[i] == 0){
        //visited == 0  I haven't visited 
                dfs(i); // Access this point 
                if(!flag)   // If after the visit ,flag Turn into false, It indicates that the ring relationship is found during the access 
                    return; // Quickly end all access 
            }
            else if(visited[i] == 1){
       //visited == 1  Explain that you have visited before , This is the second visit , That is, there is a ring relation 
                flag = false;   // Discover ring relationships ,flag Set as false
                return;
            }
        }
        visited[n] = 2; // This point is a safe point 
    }
};

Accepted
51/51 cases passed (24 ms)
Your runtime beats 33.99 % of cpp submissions
Your memory usage beats 23.46 % of cpp submissions (14.1 MB)