check_cycle_directed_bfs.cpp

#include <bits/stdc++.h>
using namespace std;

// https://practice.geeksforgeeks.org/problems/detect-cycle-in-a-directed-graph/
// https://youtu.be/iTBaI90lpDQ

class Solution{
public:
    // Function to detect cycle in a directed graph.
    bool isCyclic(int V, vector<int> adj[])
    {
        vector<int> indegree(V, 0);
        for (int i = 0; i < V; i++){
            for(auto it: adj[i])
                indegree[it]++;
        }

        queue<int> q;
        for (int i = 0; i < V; i++)
            if(indegree[i] == 0)
                q.push(i);

        vector<int> order;
        while(!q.empty())
        {
            int node = q.front();
            q.pop();
            order.push_back(node);

            for(auto it: adj[node]){
                indegree[it]--;
                if(indegree[it] == 0)
                    q.push(it);
            }
        }

        if(order.size() == V)
            return false;
        return true;
    }
};