LeetCode 0210 - Course Schedule II

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

Input: 4, [[1,0],[2,0],[3,1],[3,2]]
Output: [0,1,2,3] or [0,2,1,3]
Explanation: There are a total of 4 courses to take. To take course 3 you should have finished both     
             courses 1 and 2. Both courses 1 and 2 should be taken after you finished course 0. 
             So one correct course order is [0,1,2,3]. Another correct ordering is [0,2,1,3] .

class Solution {
public:
    vector<int> findOrder(int numCourses, vector<pair<int, int>>& prerequisites) {
        vector<int> res;
        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 vector<int>(0);
            }
            res.push_back(index);
            degrees[index] = -1;
            for(auto it : graph[index]) {
                degrees[it]--;
            }
        }
        return res;
    }
};