KosarajusAlgorithm.cs

using System.Collections.Generic;
using AlgorithmsAndDataStructures.Algorithms.Graph.Common;

namespace AlgorithmsAndDataStructures.Algorithms.Graph.Misc;

public class KosarajusAlgorithm
{
#pragma warning disable CA1822 // Mark members as static
    public bool IsConnected(WeightedGraphVertex[] graph)
#pragma warning restore CA1822 // Mark members as static
    {
        if (graph is null) return default;

        var visited = new bool[graph.Length];

        Dfs(graph, 0, visited);

        for (var i = 0; i < visited.Length; i++)
            if (!visited[i])
                return false;

        var transposedGraph = GetTransposed(graph);
        var transposedVisited = new bool[graph.Length];

        Dfs(transposedGraph, 0, transposedVisited);

        for (var i = 0; i < transposedVisited.Length; i++)
            if (!transposedVisited[i])
                return false;

        return true;
    }

    private static void Dfs(WeightedGraphVertex[] graph, int currentVertex, IList<bool> visited)
    {
        visited[currentVertex] = true;

        for (var i = 0; i < graph[currentVertex].Edges.Count; i++)
        {
            var adjacentVertex = graph[currentVertex].Edges[i].To;

            if (!visited[adjacentVertex]) Dfs(graph, adjacentVertex, visited);
        }
    }

    private static WeightedGraphVertex[] GetTransposed(IReadOnlyList<WeightedGraphVertex> graph)
    {
        var result = new WeightedGraphVertex[graph.Count];

        for (var i = 0; i < graph.Count; i++)
        {
            result[i] = result[i] == null ? new WeightedGraphVertex() : result[i];

            for (var j = 0; j < graph[i].Edges.Count; j++)
            {
                var from = graph[i].Edges[j].To;

                result[from] ??= new WeightedGraphVertex();

                result[from].Edges.Add(new WeightedGraphNodeEdge
                {
                    From = from,
                    To = graph[i].Edges[j].From,
                    Weight = graph[i].Edges[j].Weight
                });
            }
        }

        return result;
    }
}