Task: Straddling checkerboard
C#: using System;
using System.Collections.Generic;
using System.Linq;
using System.Text.RegularExpressions;

namespace StraddlingCheckerboard
{
    class Program
    {
        public readonly static IReadOnlyDictionary<char, string> val2Key;
        public readonly static IReadOnlyDictionary<string, char> key2Val;

        static Program()
        {
            val2Key = new Dictionary<char, string> {
                {'A',"30"},  {'B',"31"}, {'C',"32"},  {'D',"33"},  {'E',"5"},   {'F',"34"},  {'G',"35"},
                {'H',"0"},   {'I',"36"}, {'J',"37"},  {'K',"38"},  {'L',"2"},   {'M',"4"},   {'.',"78"},
                {'N',"39"},  {'/',"79"}, {'O',"1"},   {'0',"790"}, {'P',"70"},  {'1',"791"}, {'Q',"71"},
                {'2',"792"}, {'R',"8"},  {'3',"793"}, {'S',"6"},   {'4',"794"}, {'T',"9"},   {'5',"795"},
                {'U',"72"},  {'6',"796"},{'V',"73"},  {'7',"797"}, {'W',"74"},  {'8',"798"}, {'X',"75"},
                {'9',"799"}, {'Y',"76"}, {'Z',"77"}};

            key2Val = val2Key.ToDictionary(kv => kv.Value, kv => kv.Key);
        }

        public static string Encode(string s)
        {
            return string.Concat(s.ToUpper().ToCharArray()
                .Where(c => val2Key.ContainsKey(c)).Select(c => val2Key[c]));
        }

        public static string Decode(string s)
        {
            return string.Concat(Regex.Matches(s, "79.|7.|3.|.").Cast<Match>()
                .Where(m => key2Val.ContainsKey(m.Value)).Select(m => key2Val[m.Value]));
        }

        static void Main(string[] args)
        {
            var enc = Encode("One night-it was on the twentieth of March, 1888-I was returning");
            Console.WriteLine(enc);
            Console.WriteLine(Decode(enc));

            Console.ReadLine();
        }
    }
}

Java: import java.util.HashMap;
import java.util.Map;
import java.util.regex.*;

public class StraddlingCheckerboard {

    final static String[] keyvals = {"H:0", "O:1", "L:2", "M:4", "E:5", "S:6",
        "R:8", "T:9", "A:30", "B:31", "C:32", "D:33", "F:34", "G:35", "I:36",
        "J:37", "K:38", "N:39", "P:70", "Q:71", "U:72", "V:73", "W:74", "X:75",
        "Y:76", "Z:77", ".:78", "/:79", "0:790", "1:791", "2:792", "3:793",
        "4:794", "5:795", "6:796", "7:797", "8:798", "9:799"};

    final static Map<String, String> val2key = new HashMap<>();
    final static Map<String, String> key2val = new HashMap<>();

    public static void main(String[] args) {
        for (String keyval : keyvals) {
            String[] kv = keyval.split(":");
            val2key.put(kv[0], kv[1]);
            key2val.put(kv[1], kv[0]);
        }
        String enc = encode("One night-it was on the twentieth of March, "
                + "1888-I was returning");
        System.out.println(enc);
        System.out.println(decode(enc));
    }

    static String encode(String s) {
        StringBuilder sb = new StringBuilder();
        for (String c : s.toUpperCase().split("")) {
            c = val2key.get(c);
            if (c != null)
                sb.append(c);
        }
        return sb.toString();
    }

    static String decode(String s) {
        Matcher m = Pattern.compile("(79.|3.|7.|.)").matcher(s);
        StringBuilder sb = new StringBuilder();
        while (m.find()) {
            String v = key2val.get(m.group(1));
            if (v != null)
                sb.append(v);
        }
        return sb.toString();
    }
}