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();
        }
    }
}

C++: #include <iostream>
#include <string>
#include <map>
#include <algorithm> 
using namespace std;

class StraddlingCheckerboard
{
  map<char, string> table;
  char first[10], second[10], third[10];
  int rowU, rowV;

public:
  StraddlingCheckerboard(const string &alphabet, int u, int v)
  {
    rowU = min(u, v);
    rowV = max(u, v);

    for(int i = 0, j = 0; i < 10; ++i)
    {
      if(i != u && i != v)
      {
        first[i] = alphabet[j];
        table[alphabet[j]] = '0' + i;
        ++j;
      }

      second[i] = alphabet[i+8];
      table[alphabet[i+8]] = '0' + rowU;
      table[alphabet[i+8]] += '0' + i;

      third[i] = alphabet[i+18];
      table[alphabet[i+18]] = '0' + rowV;
      table[alphabet[i+18]] += '0' + i;
    }
  }

  string encode(const string &plain)
  {
    string out;
    for(int i = 0; i < plain.size(); ++i)
    {
      char c = plain[i];
      if(c >= 'a' && c <= 'z')
        c += 'A' - 'a';

      if(c >= 'A' && c <= 'Z')
        out += table[c];
      else if(c >= '0' && c <= '9')
      {
        out += table['/'];
        out += c;
      }
    }
    return out;
  }

  string decode(const string &cipher)
  {
    string out;
    int state = 0;
    for(int i = 0; i < cipher.size(); ++i)
    {
      int n = cipher[i] - '0';
      char next = 0;

      if(state == 1)
        next = second[n];
      else if(state == 2)
        next = third[n];
      else if(state == 3)
        next = cipher[i];
      else if(n == rowU)
        state = 1;
      else if(n == rowV)
        state = 2;
      else
        next = first[n];

      if(next == '/')
        state = 3;
      else if(next != 0)
      {
        state = 0;
        out += next;
      }
    }
    return out;
  }
};