/// <summary> /// Process the product of the count of product numbers that contains a letter 3 times and the count of product numbers that contains /// a letter 2 times. /// </summary> /// <returns>The product of the two counts.</returns> /// <param name="numbers">The list of product numbers.</param> public int Part1(string[] numbers) { int two = 0; int three = 0; foreach (string number in numbers) { PartNumber.Checks check = PartNumber.Checksum(number); if (PartNumber.HasLetterTwice(check)) { two++; } if (PartNumber.HasLetterThreeTimes(check)) { three++; } } return(two * three); }