方法一：dfs

class Solution {
public:
    vector<int> findOrder(int numCourses, vector<vector<int>>& prerequisites) {
        vector<int> res;
       vector<vector<int>> graph(numCourses,vector<int>(0));
        vector<int>indegree(numCourses,0);
        queue<int> que;
        for(auto vec:prerequisites)
        {
            graph[vec[1]].push_back(vec[0]);
            ++indegree[vec[0]];
        }
        for(int i=0;i<numCourses;i++)
        {
            if(indegree[i]==0)
            {
                que.push(i);
            }
        }
        while(!que.empty())
        {
            int second=que.front();
            que.pop();
            res.push_back(second);
            --numCourses;
            for(auto first:graph[second])
            {
                --indegree[first];
                if(indegree[first]==0)
                {
                    que.push(first);
                }
            }
        }
        if(numCourses!=0)
        {
            res.clear();
        }
        return res;
    }
};