class Solution {
public:
    struct GraphNode
    {
        int lable;
        vector<GraphNode*> neighbors;
        GraphNode(int x):lable(x){}
    };
    
    bool DFS_graph(GraphNode* node, vector<int>& visit)
    {
        //正在訪問
        visit[node->lable] = 0;
        for(int i=0
 
; i<node->neighbors.size(); ++i)
        {
            if(visit[node->neighbors[i]->lable] == -1)
            {
                //在訪問的裏面已經返回false就直接返回false了
                if(DFS_graph(node->neighbors[i], visit) == 0)
                {
                    return false;
                }
            }
            
            
 
// 第一個遇到有環的
            if(visit[node->neighbors[i]->lable] == 0)
            {
                return false;
            }
        }
        //完成了以後
        visit[node->lable] = 1;
        return true;
    }
    
    bool canFinish(int numCourses, vector<pair<int, int>>& prerequisites) {
        vector
 
<GraphNode*> graph;
        vector<int> visit;//節點狀態，-1表示沒有訪問，0表示正在訪問，1表示已經訪問
        bool flag = true;
        for(int i=0; i<numCourses; ++i)
        {
            graph.push_back(new GraphNode(i));
        }
        visit.resize(numCourses, -1);
        
        for(int i=0; i<prerequisites.size(); ++i)
        {
            GraphNode* begin = graph[prerequisites[i].second];
            GraphNode* end = graph[prerequisites[i].first];
            begin->neighbors.push_back(end);
        }
        
        for(int i=0; i<graph.size(); ++i)
        {
            if(visit[i] == -1 && !DFS_graph(graph[i], visit))
            {
                flag = false;
                break;
            }
        }
        
        for(int i=0; i<numCourses; ++i)
        {
            delete graph[i];
        }
        return flag;
    }
};

 bool canFinish(int numCourses, vector<pair<int, int>>& prerequisites) {
        vector<GraphNode*> graph;
        vector<int> degree;
            
        for(int i=0; i<numCourses; ++i)
        {
            graph.push_back(new GraphNode(i));
        }
        degree.resize(numCourses, 0);
        
        for(int i=0; i<prerequisites.size(); ++i)
        {
            GraphNode* begin = graph[prerequisites[i].second];
            GraphNode* end = graph[prerequisites[i].first];
            begin->neighbors.push_back(end);
            ++degree[prerequisites[i].first];
        }
        queue<GraphNode*> queue;
        for(int i=0; i<numCourses; ++i)
        {
            if(degree[i] == 0)
            {
                queue.push(graph[i]);
            }
        }
            
        while(!queue.empty())
        {
            auto *node = queue.front();
            queue.pop();
            for(int i=0; i<node->neighbors.size(); ++i)
            {
                --degree[node->neighbors[i]->lable];
                if(degree[node->neighbors[i]->lable] == 0)   
                {
                    queue.push(node->neighbors[i]);
                }  
            }
        }
        for(int i=0; i<numCourses; ++i )
            delete graph[i];
        
        for(int i=0; i<degree.size(); ++i)
        {
            if(degree[i])
                return false;
        }
        return true;
    }
};

Leetcode 207. 課程表