leetcode210. 课程表 II

题目链接：https://leetcode-cn.com/problems/course-schedule-ii/

1. 思路：拓扑排序，注意建图初始化

class Solution {
public:
    vector<vector<int>>map;
    vector<int>index;
    queue<int>q;
public:
    vector<int> findOrder(int numCourses, vector<vector<int>>& prerequisites) {
        vector<int>ans;
        map.resize(numCourses);
        index.resize(numCourses);
        //建图
        for(int i=0;i<prerequisites.size();i++)
        {
            int x=prerequisites[i][1];
            int y=prerequisites[i][0];
            //cout<<x<<"+"<<y<<endl;
            map[x].push_back(y);
            index[y]++;
        }

        //遍历
        for(int i=0;i<index.size();i++)
        {
            if(index[i]==0)
            {
                q.push(i);
            }
        }
        while(!q.empty())
        {
            int k=q.front();
            //cout<<k<<endl;
            q.pop();
            ans.push_back(k);
            for(int i=0;i<map[k].size();i++)
            {
                index[map[k][i]]--;
                if(index[map[k][i]]==0)
                {
                    q.push(map[k][i]);
                }
            }
        }
        if(ans.size()!=numCourses)
        {
            ans.clear();
            return ans;
        }
        return ans;
    }
};