course schedule 2

1）利用indegree记录每一个课程要多少个条件；

2）每次选取所需条件为0的课学习，并对indegree表进行update；

3）知道indegree表变为0；

class Solution {
public:
    vector<int> findOrder(int numCourses, vector<pair<int, int>>& prerequisites) {

        vector<int> seq;

        if(numCourses == 0)
        {
            return seq;
        }

        map<int, int> ind;
        for(int i = 0; i<numCourses; i++)
        {
            ind[i] = 0;
        }

        for(pair<int, int> tmp : prerequisites)
        {
            ind[tmp.first]++;
        }

        bool circle = false;

        while(ind.size() != 0 && !circle)
        {
            circle = true;
            int courseno = -1;

            for(auto& kv : ind)
            {
                if(kv.second != 0)
                {
                    continue;
                }

                circle = false;
                courseno = kv.first;
                break;
            }

            if(circle)
            {
                vector<int> circleSeq;
                return circleSeq;
            }

            ind.erase(courseno);
            seq.push_back(courseno);
            for(pair<int, int> tmp : prerequisites)
            {
                if(tmp.second == courseno)
                {
                    ind[tmp.first]--;
                }
            }
        }

        return seq;

    }
};