LeetCode 0207 - Course Schedule

Input: 2, [[1,0]] 
Output: true
Explanation: There are a total of 2 courses to take. 
             To take course 1 you should have finished course 0. So it is possible.

Input: 2, [[1,0],[0,1]]
Output: false
Explanation: 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.

class Solution {
public:
    bool canFinish(int numCourses, vector<pair<int, int>>& prerequisites) {
        vector<unordered_set<int>> graph(numCourses);
        for(auto it : prerequisites) {
            graph[it.second].insert(it.first);
        }
        vector<int> degrees(numCourses, 0);
        for(auto it : graph) {
            for(auto index : it) {
                degrees[index]++;
            }
        }
        for(int i = 0; i < numCourses; i++) {
            int index = 0;
            for(; index < numCourses; index++) {
                if(degrees[index] == 0) {
                    break;
                }
            }
            if(index == numCourses) {
                return false;
            }
            degrees[index] = -1;
            for(auto it : graph[index]) {
                degrees[it]--;
            }
        }
        return true;
    }
};