public Letters Subtract(Letters second) { string letters1 = characters; string letters2 = second.characters; string different_letters = ""; while (true) { if (letters2.Length == 0) return new Letters(different_letters + letters1); if (letters1.Length == 0) return null; { char letter2 = letters2[0]; char letter1 = letters1[0]; if (letter1 > letter2) return null; if (letter1 < letter2) { letters1 = letters1.Substring(1); different_letters += letter1; continue; } //if (letter1 == letter2) throw new Exception("Error: letter1 == letter2"); letters1 = letters1.Substring(1); letters2 = letters2.Substring(1); } } }
private static string Subtract(string first, string sencod) { Letters letters1 = new Letters(first); Letters letters2 = new Letters(sencod); Letters different_letters = letters1.Subtract(letters2); if (different_letters == null) return null; return different_letters.ToString(); }