拓扑排序：课程表Course Schedule2

https://leetcode.com/problems/course-schedule-ii/

class Solution {public:    vector<int> findOrder(int numCourses, vector<pair<int, int>>& prerequisites) {        vector<int> res;        vector<int> inDgree(numCourses,0);        map<int, vector<int> >adjNode;        int len = prerequisites.size();        for (int i = 0; i < len; i++){            pair<int, int> p = prerequisites[i];            //自环            if (p.first == p.second)                return res;            //平行边            if (find(adjNode[p.second].begin(), adjNode[p.second].end(), p.first) == adjNode[p.second].end()){                adjNode[p.second].push_back(p.first);                inDgree[p.first]++;            }        }        queue<int> Q;        for (int i=0; i < numCourses; i++){            if (inDgree[i] == 0)                Q.push(i);        }        while (!Q.empty()){            int front = Q.front();            Q.pop();            res.push_back(front);            vector<int> adj = adjNode[front];            for (int i:adj){                inDgree[i]--;                if (inDgree[i] == 0){                    Q.push(i);                }            }        }        if (res.size() == numCourses){            return res;        }        else{            return vector<int>();        }    }};