Adv2018/D5.1/Program.cs

using System;
using System.IO;

namespace D5._1
{
    class Program
    {
        static void Main(string[] args)
        {
            if (args.Length < 1) return;
            if (File.Exists(args[0]) == false) return;

            var file = File.OpenText(args[0]);

            string polymer = file.ReadLine();
            Reduce(ref polymer);

            Console.WriteLine(polymer.Length);
        }

        static void Reduce(ref string polymer)
        {
            for (int i = 1; i < polymer.Length; i++)
            {
                char cc = polymer[i];
                char cm = polymer[i-1];

                if (Math.Abs(cc - cm) <= 26) continue;
                if (Up(cc) == Up(cm))
                {
                    polymer = polymer.Remove(i - 1, 2);
                    i = Math.Max(0, i - 2);
                }
            }
        }

        static char Up(char c)
        {
            if (c >= 'A' && c <= 'Z') return c;
            return (char) ('A' + (c - 'a')); 
        }
    }
}