1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44

#include <iostream>
#include <vector>
#include <queue>

class Solution {
public:
    std::vector<int> findOrder(int numCourses, std::vector<std::vector<int>>& prerequisites) {
        std::vector<int> res;
        std::vector<int> inDegree(numCourses, 0);
        std::vector<std::vector<int>> adj(numCourses);
        for (auto& e : prerequisites) {
            adj[e[1]].push_back(e[0]);
        }
        for (auto& course : adj) {
            for (auto& v : course) {
                ++inDegree[v];
            }
        }
        
        std::queue<int> q;
        for (int i = 0; i < numCourses; ++i) {
            if (inDegree[i] == 0) {
                q.push(i);
            }
        }
        
        while (!q.empty()) {
            int u = q.front();
            q.pop();
            res.push_back(u);
            for (auto& v : adj[u]) {
                --inDegree[v];
                if (inDegree[v] == 0) {
                    q.push(v);
                }
            }
        }
        
        if (res.size() != numCourses) {
            return {};
        }
        return res;
    }
};