[LeetCode] Course Schedule II

Well, this problem is spiritually similar to to Course Schedule. You only need to store the nodes in the order you visit into a vector during BFS or DFS. Well, for DFS, a final reversal is required.

---

BFS

class Solution {
public:
    vector<int> findOrder(int numCourses, vector<pair<int, int>>& prerequisites) {
        vector<unordered_set<int>> graph = make_graph(numCourses, prerequisites);
        vector<int> degrees = compute_indegree(graph);
        queue<int> zeros;
        for (int i = 0; i < numCourses; i++)
            if (!degrees[i]) zeros.push(i);
        vector<int> toposort;
        for (int i = 0; i < numCourses; i++) {
            if (zeros.empty()) return {};
            int zero = zeros.front();
            zeros.pop();
            toposort.push_back(zero);
            for (int neigh : graph[zero]) {
                if (!--degrees[neigh])
                    zeros.push(neigh);
            }
        }
        return toposort;
    }
private:
    vector<unordered_set<int>> make_graph(int numCourses, vector<pair<int, int>>& prerequisites) {
        vector<unordered_set<int>> graph(numCourses);
        for (auto pre : prerequisites)
            graph[pre.second].insert(pre.first);
        return graph; 
    }
    vector<int> compute_indegree(vector<unordered_set<int>>& graph) {
        vector<int> degrees(graph.size(), 0);
        for (auto neighbors : graph)
            for (int neigh : neighbors)
                degrees[neigh]++;
        return degrees;
    }
};

---

DFS

class Solution {
public:
    vector<int> findOrder(int numCourses, vector<pair<int, int>>& prerequisites) {
        vector<unordered_set<int>> graph = make_graph(numCourses, prerequisites);
        vector<int> toposort;
        vector<bool> onpath(numCourses, false), visited(numCourses, false);
        for (int i = 0; i < numCourses; i++)
            if (!visited[i] && dfs(graph, i, onpath, visited, toposort))
                return {};
        reverse(toposort.begin(), toposort.end());
        return toposort;
    }
private:
    vector<unordered_set<int>> make_graph(int numCourses, vector<pair<int, int>>& prerequisites) {
        vector<unordered_set<int>> graph(numCourses);
        for (auto pre : prerequisites)
            graph[pre.second].insert(pre.first);
        return graph;
    }
    bool dfs(vector<unordered_set<int>>& graph, int node, vector<bool>& onpath, vector<bool>& visited, vector<int>& toposort) { 
        if (visited[node]) return false;
        onpath[node] = visited[node] = true; 
        for (int neigh : graph[node])
            if (onpath[neigh] || dfs(graph, neigh, onpath, visited, toposort))
                return true;
        toposort.push_back(node);
        return onpath[node] = false;
    }
};