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class Solution { public: vector<int> findOrder(int n, vector<vector<int>>& pre) { vector<int> res; vector<vector<int>> adj(n); vector<int>deg(n,0); for(int i=0;i<pre.size();i++){ adj[pre[i][1]].push_back(pre[i][0]); deg[pre[i][0]]++; } queue<int>s; for(int i=0;i<n;i++){ if(deg[i]==0) s.push(i); } while(!s.empty()){ int f=s.front(); res.push_back(f); s.pop(); n--; for(auto nbr:adj[f]){ deg[nbr]--; if(deg[nbr]==0) s.push(nbr); } } vector<int> empty; if(n==0) return res; else return empty; } };