Leetcode 207 Course Schedule

Q:

There are a total of n courses you have to take, labeled from 0 to n - 1.

Some courses may have prerequisites, for example to take course 0 you have to first take course 1, which is expressed as a pair:[0,1]

Given the total number of courses and a list of prerequisite pairs, is it possible for you to finish all courses?

For example:

2, [[1,0]]

There are a total of 2 courses to take. To take course 1 you should have finished course 0. So it is possible.

2, [[1,0],[0,1]]

There are a total of 2 courses to take. To take course 1 you should have finished course 0, and to take course 0 you should also have finished course 1. So it is impossible.

Note:

1. The input prerequisites is a graph represented by a list of edges, not adjacency matrices. Read more about how a graph is represented.
2. You may assume that there are no duplicate edges in the input prerequisites.

A:

class Solution {
public:
    bool canFinish(int numCourses, vector<pair<int, int>>& prerequisites) {
        if (numCourses == 0 || prerequisites.empty()){
            return true;
        }
        
        graph = vector<vector<int> >(numCourses);
        vis = vector<int>(numCourses, 0);
        
        for (auto i : prerequisites) {
            graph[i.second].push_back(i.first);
        }
        
        for (int u = 0; u < numCourses; ++u) {
            if (0 == vis[u] && !dfs(u))
                return false;
        }
        return true;
    }
    
private:
    vector<vector<int> > graph;
    vector<int> vis;
    
    bool dfs(int u) {
        vis[u] = 1;
        for (auto v : graph[u]) {
            if (vis[v] == 1){
                return false;
            }
            if (dfs(v) == false){
                return false;
            }
        }
        
        vis[u] = 2;
        return true;
    }
};