方法：拓扑排序—后序遍历+反转

    1. class Solution {
    2. public:
    3.     vector<bool> visited,visit;
    4.     vector<bool> inpath;
    5.     bool iscycle=false;
    6.     vector<int> result;
    7.     vector<int> findOrder(int numCourses, vector<vector<int>>& prerequisites) {
    8.         vector<vector<int>>graph= buildgraph(numCourses,prerequisites);
    9.         visit.resize(numCourses);
    10.         if(check(numCourses,graph)){
    11.             return result;
    12.         }
    13.         for(int i=0;i<numCourses;i++){
    14.             travel(i,graph);
    15.         }
    16.         reverse(result.begin(), result.end());
    17.         return result;
    18.     }
    19.     bool check(int numCourses, vector<vector<int>>& graph){
    20.         visited.resize(numCourses);
    21.         inpath.resize(numCourses);
    22.         for(int i=0;i<numCourses;i++){
    23.             dfs(i,graph);
    24.         }
    25.         return iscycle;
    26.     }
    27.     vector<vector<int>> buildgraph(int numCourses, vector<vector<int>>& prerequisites){
    28.         vector<vector<int>> graph;
    29.         graph.resize(numCourses);
    30.         for(auto prere:prerequisites){
    31.             int from=prere[1];
    32.             int to=prere[0];
    33.             graph[from].emplace_back(to);
    34.         }
    35.         return graph;
    36.     }
    37.     void dfs(int num, vector<vector<int>>& graph){
    38.         if(inpath[num]){
    39.             iscycle=true;
    40.         }
    41.         if(visited[num]||iscycle){
    42.             return;
    43.         }
    44.         visited[num]=true;
    45.         inpath[num]=true;
    46.         for(auto g:graph[num]){
    47.             dfs(g,graph);
    48.         }
    49.         inpath[num]=false;
    50.     }
    51.     void travel(int num, vector<vector<int>>& graph){
    52.         if(visit[num]){
    53.             return;
    54.         }
    55.         visit[num]=true;
    56.         for(auto g:graph[num]){
    57.             travel(g,graph);
    58.         }
    59.         result.emplace_back(num);
    60.     }
    61. };