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class Solution {
public:
    vector<int> findOrder(int numCourses, vector<pair<int, int>>& prerequisites) {
        vector<int> result;
        if (numCourses == 1) {
            result.push_back(0);
            return result;
        }
        
        vector<vector<int> > map(numCourses, vector<int>());
        stack<int> reversePos;
        
        for(int i = 0; i < prerequisites.size(); i++) {
            map[prerequisites[i].second].push_back(prerequisites[i].first);
        }
        vector<bool> isVisited(numCourses, false);
        for (int i = 0; i < numCourses; ++i) {
            if (!isVisited[i]) {
                vector<bool> onStack(numCourses, false);
                if (hasCycle(map, i, isVisited, onStack, reversePos)) return result;
            }
        }
        
        while (!reversePos.empty()) {
            int index = reversePos.top();
            reversePos.pop();
            result.push_back(index);
        }
        return result;
    }
    
    bool hasCycle(vector<vector<int>> &map, int i, vector<bool> &isVisited, vector<bool> &onStack, 
            stack<int> &reversePos) {
        isVisited[i] = true;
        onStack[i] = true;
        for (int k : map[i]) {
            if (onStack[k])
                return true;
            else if (!isVisited[k])
                if (hasCycle(map, k, isVisited, onStack, reversePos))
                    return true;
        }
        onStack[i] = false;
        reversePos.push(i);
        return false;
    }
};