(C#) Project Euler 079 - Passcode Derivation

/// A common security method used for online banking is to ask the user for three random characters from a passcode.
/// For example, if the passcode was 531278, they may ask for the 2nd, 3rd, and 5th characters; the expected reply would be: 317.
/// 
/// The text file, keylog.txt, contains fifty successful login attempts.
/// 
/// Given that the three characters are always asked for in dorder, analyse the file so as to determine the shortest possible secret passcode of unknown length.

 

 

using System;

class Program
{
    static void Main()
    {
        string path = @"keylog.txt";
        string[] file = File.ReadAllLines(path);

        Dictionary<char, HashSet<char>> graph = new Dictionary<char, HashSet<char>>();

        foreach (string line in file)
        {
            for (int i = 0; i < line.Length; i++)
            {
                char current = line[i];

                if (!graph.ContainsKey(current))
                {
                    graph[current] = new HashSet<char>();
                }

                for (int j = i + 1; j < line.Length; j++)
                {
                    char next = line[j];
                    graph[current].Add(next);
                }
            }
        }

        HashSet<char> visited = new HashSet<char>();
        List<char> result = new List<char>();

        foreach (char node in graph.Keys)
        {
            if (!visited.Contains(node))
            {
                TopologicalSort(graph, node, visited, result);
            }
        }

        result.Reverse();

        string passcode = string.Join("", result);
        Console.WriteLine($"Shortest possible secret passcode: {passcode}");
    }

    static void TopologicalSort(Dictionary<char, HashSet<char>> graph, char node, HashSet<char> visited, List<char> result)
    {
        visited.Add(node);

        if (graph.ContainsKey(node))
        {
            foreach (char neighbor in graph[node])
            {
                if (!visited.Contains(neighbor))
                {
                    TopologicalSort(graph, neighbor, visited, result);
                }
            }
        }

        result.Add(node);
    }
}