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Prashant Mishra
Prashant Mishra

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Strongly connected components in the graph

You are given an unweighted directed graph of 'V' vertices and 'E' edges. Your task is to print strongly connected components (SCCs) present in the graph.

Input format:
You are given 'n' which is no. of vertices in the graph(0-based indexed)
'edges' 2d array of size m*2, where m is no. of edges and 2 is two vertices between which the edge exists. That is edges[i][0]---->edges[i][1] there is an edge between node 0 and 1.

TC: topo sort = O(N+E) +
transpose of the graph = O(N+E) +
dfs on topo sort with transpose graph = O(N+E)

import java.util.*;
public class Solution {

    public static List<List<Integer>> stronglyConnectedComponents(int n, int[][] edges) {
        List<List<Integer>> list = new ArrayList<>();
        // Write your code here
        // we identify which are the strogly conected component,
        // strongly connected components are the components where all the
        //are reachable by every other node in the component.
        //steps to solve this using kosaraju's SCC
        //1. find toposort of the graph.
        //2. transpose the graph(change direction of the edges)
        //3. do dfs on the toposort with the transpose graph.
        //creating adjacency list
        ArrayList<ArrayList<Integer>> adj = new ArrayList<>();
        ArrayList<ArrayList<Integer>> tadj  = new ArrayList<>();
        for(int i =0;i<n;i++) {
            adj.add(new ArrayList<>());
            tadj.add(new ArrayList<>());
        for(int ed [] : edges) {
            //creating adjacency graph
            ArrayList<Integer> temp = adj.get(ed[0]);
            //creating transpose adjacency graph
            ArrayList<Integer> temp2 = tadj.get(ed[1]);

        int visited[] = new int[n];
        Stack<Integer> stack = new Stack<>();
        for(int i =0;i<n;i++){
        //now stack has topo sorted nodes in it.
           int node   = stack.pop();
           if(visited[node] ==0){

               ArrayList<Integer> connComp = new ArrayList<>();
        return list;
     public static void dfsOnTransposeGraph(int node, int[] visited, 
                          ArrayList<ArrayList<Integer>> adj,ArrayList<Integer> connComp){
        visited[node] = 1;
        for(Integer it : adj.get(node)){
            if(visited[it]==0) {
    public static void dfs(int node, int[] visited, Stack<Integer> stack, 
                          ArrayList<ArrayList<Integer>> adj){
        visited[node] = 1;
        for(Integer it : adj.get(node)){
            if(visited[it]==0) dfs(it,visited,stack,adj);
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