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//Title of this code
//Rextester.Program.Main is the entry point for your code. Don't change it.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text.RegularExpressions;

namespace Rextester
{
    public class Program
    {
        //Not sure if "static" would count toward the golf score, so I'll just make this an instance method
        double f(int x){return 2>x?1:x*f(x-1);}
        
        public static void Main(string[] args)
        {
            //I can't resist: 
            //Yo dawg, I heard you like programs so we put a Program in your Program so you can Program while you program
            var instance = new Program();
            
            Console.WriteLine("n\tn!\texpected");
            
            for(var i = 0; i <= 5; i++)
            {
                instance.PrintResult(i);
            }
            
            Console.WriteLine("...");
            Console.WriteLine("n\tn!\t\t\texpected");
            
            instance.PrintResult(125);
        }
         
        //Extras: Check the math and display the result
        void PrintResult(int num)
        {
            var result = f(num);
            var expectedResult = GetExpected(num);
            var ok = result == expectedResult;
            
            Console.WriteLine("{0}\t{1}\t{2}\t {3}", num, result, expectedResult, ok ? "ok" : "ERROR");
        }
        
        static double GetExpected(int num)
        {
            if (num < 0) throw new ArgumentOutOfRangeException();
            if (num == 0) return 1;
            return num * GetExpected(num - 1);
        }            
    }
}

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Compilation time: 0.05 sec, absolute running time: 0.06 sec, cpu time: 0.03 sec, average memory usage: 16 Mb, average nr of threads: 4