kahn's_algorithm

// Given a Directed Acyclic Graph (DAG) with V 
// vertices and E edges.
// Find any Topological Sorting of that Graph.

vector<int> topoSort(int V, vector<int> adj[]) 
  {
      int indeg[V];
      memset(indeg,0,sizeof(indeg));
      
      for(int i=0;i<V;i++)
      {
          for(auto it:adj[i])
          {
              indeg[it]++;
          }
      }
      queue<int>q;
      for(int i=0;i<V;i++)
      {
          if(indeg[i]==0)
          {
              q.push(i);
          }
      }
      vector<int>v;
      while(!q.empty())
      {
          int b=q.size();
          while(b--)
          {
              int node=q.front();
              q.pop();
              v.push_back(node);
              for(int child:adj[node])
              {
                  if(--indeg[child]==0)
                  {
                      q.push(child);
                  }
              }
          }
      }
      return v;
  }

  // time complexity- O(V+E)
  // space complexity- O(V)