1136

class Solution {
public:
    int minimumSemesters(int n, vector<vector<int>>& relations) {
        vector<int> indegree(n+1);
        vector<vector<int>> graph(n+1);
        for(auto edge: relations){
            graph[edge[0]].push_back(edge[1]);
            indegree[edge[1]]++;
        }

        int level = 0, count=0;
        queue<int> q;
        for(int i=1; i<=n; i++){
            if(!indegree[i])
                q.push(i);
        }

        while(!q.empty()){
            int sz = q.size();
            count += sz;
            for(int i=0; i<sz; i++){
                int curr = q.front(); q.pop();
                for(int next: graph[curr]){
                    if(--indegree[next] == 0)
                        q.push(next);
                }
            }
            level++;
        }
        return (count == n) ? level:-1;
    }
};