using System;

using System.Collections;       

using System.Linq;

​

public class Program

{

    public static void Main()

    {

        Console.WriteLine("{0}", isIsogram("abcdefg"));

        Console.WriteLine("{0}", isIsogram("abCcdef"));

        Console.WriteLine("{0}", isIsogram("bacb"));

        Console.WriteLine("{0}", isIsogram("fgabb"));

        Console.WriteLine("Max occurrence: {0}", maxOccurrence("Hello World"));

    }

    public static Char maxOccurrence(string w){

        return w.ToLower().GroupBy(x => x).Select(g => new { Symbol = g.Key, Count = g.Count() }).OrderByDescending(x => x.Count).First().Symbol;

        //------

        char r;

        int o = 0;

        var ww = w.ToList();

        var ch = ww.Distinct();

        foreach (var chr in ch){

            int oc = ww.Where(x => x == chr).Count();

            if (oc > 1 && oc > o){

                o = oc;

                r = chr;

            }

        }

        return r;

    }

    public static bool isIsogram(string w) {

        return w.ToLower().GroupBy(x => x).Select(g => new { Symbol = g.Key, Count = g.Count() }).Any(x => x.Count > 1);

        // ----------

        bool r = false;

        //var arr = w.ToCharArray();

        for (int i = 0; i < w.Length - 1 && !r; i++) {

            string l = w.Substring(i,1);

            if ( w.ToLower().IndexOf(l.ToLower(),i + 1) > -1) {

                r = true;               

            }

        }

        return r;       

    }

}