Program.cs

using System;
using System.Collections.Generic;
using System.Linq;

namespace Day9
{
    class Program
    {
        static void Main(string[] args)
        {
            int[][] Map;

            List<int[]> MapAsList = new List<int[]>();
            foreach (String line in System.IO.File.ReadLines(@"c:\Temp\day9.txt"))
            {             
                MapAsList.Add(Array.ConvertAll(line.ToCharArray(), x => (int)Char.GetNumericValue(x)));
            }

            Map = MapAsList.ToArray();

            List<int> LowPoints = new List<int>();

            int Y = 0;
            List<int> Basins = new List<int>();

            foreach (int[] Line in Map)
            {
                int X = 0;
                foreach (int Candidate in Line)
                {
                    if (IsLowPoint(Y, X, Map))
                    {
                        LowPoints.Add(Candidate + 1);
                        int Basin = CountNeighbors(Y, X, Map);
                        Basins.Add(Basin);
                    }
                    X++;
                }
                Y++;
            }
            Console.WriteLine("Part 1: " + LowPoints.Sum());
            int[] top3 = Basins.OrderByDescending(x => x).Take(3).ToArray();
            Console.WriteLine("Part 2: " + top3[0] * top3[1] * top3[2]);

        }

        static int CountNeighbors(int Y, int X, int[][] Map)
        {
            if (X < 0 || Y < 0)
            {
                return 0;
            }

            Map[Y][X] = 9;

            int ret = 1;

            // N
            if (Y > 0 && Map[Y-1][X] != 9) {
               ret = ret + CountNeighbors(Y - 1, X, Map);  
            }

            // W
            if (X > 0 && Map[Y][X-1] != 9)
            {
                ret = ret + CountNeighbors(Y, X - 1, Map);
            }

            // E
            if (X < Map[Y].Length -1 && Map[Y][X + 1] != 9)
            {
                ret = ret + CountNeighbors(Y, X + 1, Map);
            }

            // S 
            if (Y < Map.Length - 1 && Map[Y + 1][X] != 9)
            {
                ret = ret + CountNeighbors(Y + 1, X, Map);
            }

            return ret;
        }

        static bool IsLowPoint(int Y, int X, int[][] Map)
        {
            List<int> TestNumbers = new List<int>();

            // Top row
            if (Y != 0)
            {
                if (X > 0)
                {
                    TestNumbers.Add(Map[Y - 1][X - 1]);
                }
                TestNumbers.Add(Map[Y - 1][X]);

                if (X < Map[Y -1].Length -1)
                {
                    TestNumbers.Add(Map[Y - 1][X + 1]);
                }
            }

            // Row

            if (X > 0)
            {
                TestNumbers.Add(Map[Y][X - 1]);
            }

            if (X < Map[Y].Length - 1)
            {
                TestNumbers.Add(Map[Y][X + 1]);
            }

            // Bottom row
            if (Y < Map.Length - 1)
            {
                if (X > 0)
                {
                    TestNumbers.Add(Map[Y + 1][X - 1]);
                }
                TestNumbers.Add(Map[Y + 1][X]);

                if (X < Map[Y].Length - 1)
                {
                    TestNumbers.Add(Map[Y + 1][X + 1]);
                }

            }

            foreach(int TestNumber in TestNumbers)
            {
                if (TestNumber <= Map[Y][X]) return false;
            }

            return true;

        }
    }
}