leetcode 207课程表

 

class Solution {
public:bool canFinish(int numCourses, vector<vector<int>>& prerequisites) {//验证是否为DAG，每次验证指向的是否已经存在于当前图中//建图vector<int> indegree(numCourses,0);//入度vector<vector<int>> graph(numCourses,vector(0,0));//邻接表for(int i=0;i<prerequisites.size();i++){indegree[prerequisites[i][1]]++;graph[prerequisites[i][0]].push_back(prerequisites[i][1]);}//显示图showgraph(graph,numCourses);//BFSqueue<int> q;for(int i=0;i<numCourses;i++){if(indegree[i]==0){q.push(i);}}//将入度为0的点且未访问过的进入set，然后将其后继的入度全部减一，循环执行int cnt=0;while(!q.empty()){int front=q.front();q.pop();cnt++;for(int j=0;j<graph[front].size();j++){int v=graph[front][j];indegree[v]--;if(indegree[v]==0){q.push(v);}}}return cnt==numCourses;//当有环时，总有一些点入度不能减到0，因此不能完全bfs遍历
    }
private:void showgraph(vector<vector<int>>& graph,int numCourses){for(int i=0;i<numCourses;i++){cout<<i<<": ";for(int j=0;j<graph[i].size();j++){cout<<graph[i][j]<<",";}cout<<endl;}}
};