using System;

using System.Collections.Generic;

using System.Linq;

​

using static System.Console;

public class Program

{

    public static List<string> Commands { get; } =

        new List<string>

        {

            "alias",

            "exec",

            "help",

            "install",

            "list",

            "run",

            "setup",

            "uninstall",

            "update-self",

            "upgrade",

            "use",

            "version",

            "HelloWorld",

            "HelloWorldWideWeb",

            "HelloPeter"

        };

    public static void Main()

    {

        WriteLine(Expand("al"));

        WriteLine(Expand("EXE"));

        WriteLine(Expand("up"));

        WriteLine(Expand("h"));

        WriteLine(Expand("hell"));

        WriteLine(Expand("hellow"));

        WriteLine(Expand("HELLOP"));

    }

    public static string Expand(string input)

    {

        var candidates = Commands.Where(x => x.StartsWith(input, StringComparison.CurrentCultureIgnoreCase)).ToList();

        if (candidates.Count == 0)

            return input;

        if (candidates.Count == 1)

            return candidates.First();

        var commonPrefix = CommonPrefix(candidates);

        return commonPrefix.ToLowerInvariant();

    }

    public static string CommonPrefix(IList<string> options)

    {

        if (options.Count == 0)

            return string.Empty;

        if (options.Count == 1)

            return options[0];

        int prefixLength = 0;

        foreach (char c in options[0])

        {

            foreach (string s in options)

            {

                if (s.Length <= prefixLength || char.ToUpperInvariant(s[prefixLength]) != char.ToUpperInvariant(c))

                {

                    return options[0].Substring(0, prefixLength);

                }

            }

            prefixLength++;

        }

        return options[0];

    }

}