Leetcode：207. Course Schedule

代码：

class Solution {

public:

    bool canFinish(int numCourses, vector<pair<int, int>>& prerequisites) {

        vector<unordered_set<int>> graph = make_graph(numCourses, prerequisites);

        vector<bool> onpath(numCourses, false), visited(numCourses, false);

        for (int i = 0; i < numCourses; i++)

            if (!visited[i] && dfs_cycle(graph, i, onpath, visited))

                return false;

        return true;

    }

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_cycle(vector<unordered_set<int>>& graph, int node, vector<bool>& onpath, vector<bool>& visited) {

        if (visited[node]) return false;

        onpath[node] = visited[node] = true; 

        for (int neigh : graph[node])

            if (onpath[neigh] || dfs_cycle(graph, neigh, onpath, visited))

                return true;

        return onpath[node] = false;

    }
};

思路：

对图进行一次深度优先搜索，如果存在某个点通过搜素可以搜索到该点本身，则证明图中存在环，否则就是有向无环图