allDifferent.cs

using System;
using System.Collections.Generic;
using System.Linq;

namespace Different
{
    class Program
    {
        private static bool AllDifferent(IEnumerable<int> xs)
        {
            return AllDifferentHelper(xs, x => false);
        }
        private static bool AllDifferentLoop(IEnumerable<int> xs)
        {
            Func<int,bool> func = x => false;
            while (true)
            {
                if (!xs.Any()) return true;
                var current = xs.First();
                if (func(current)) return false;
                xs = xs.Skip(1);
                var func1 = func;
                func = x => (x == current) || func1(x);
            }
        }
        private static bool AllDifferentHelper(IEnumerable<int> xs, Func<int, bool> func)
        {
            if (!xs.Any()) return true;
            var current = xs.First();
            return !func(current) && AllDifferentHelper(xs.Skip(1), x => (x == current) || func(x));
        }
        static void Main()
        {
            var list1 = new List<int> {1, 3, 5, 6, 7, 9, 11};
            var list3 = new List<int> {1, 1};
            Console.WriteLine(AllDifferent(list1));
            Console.WriteLine(AllDifferent(list3));
            Console.WriteLine(AllDifferentLoop(list1));
            Console.WriteLine(AllDifferentLoop(list3));

        }
    }
}