Program.cs

// SPDX-License-Identifier: GPL-3.0-or-later
// Copyright © 2020 Aman Agnihotri

using System;
using System.Collections.Generic;
using System.Collections.Immutable;
using System.IO;
using System.Linq;
using MoreLinq;

var jolts = File.ReadLines("Input.txt")
  .Select(int.Parse)
  .ToImmutableSortedSet();

jolts = jolts.Add(0).Add(jolts.Max + 3);

var diffCounts = jolts
  .Window(2)
  .GroupBy(js => js[^1] - js[0])
  .ToImmutableDictionary(diff => diff.Key, diff => diff.Count());

var routes = new Dictionary<int, long> {{0, 1}};

jolts.Skip(1).ForEach(jolt =>
{
  routes[jolt] = routes.GetValueOrDefault(jolt - 1) +
                 routes.GetValueOrDefault(jolt - 2) +
                 routes.GetValueOrDefault(jolt - 3);
});

Console.WriteLine(diffCounts[1] * diffCounts[3]);
Console.WriteLine(routes[jolts.Max]);