Денис

using System;
using System.IO;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace Slovar
{
    class Program
    {
        static void Main(string[] args)
        {
            //SortedDictionary<string, string> dict = new SortedDictionary<string, string>();
            //dict.Add("byte", "byte");
            SortedSet<string> dict = new SortedSet<string>();
            //List<string> dict = new List<string>();
            dict.Add("byte");
            StringBuilder result = new StringBuilder();
            using (var sr = new StreamReader("input.txt"))
            {
                while (!sr.EndOfStream)
                {
                    string str = sr.ReadLine();
                    //Console.WriteLine(str);
                    if (str[0] == '+')
                    {
                        string temp = str.Substring(1, str.Length - 1);
                        if(!dict.Contains(temp))
                        {
                            dict.Add(temp);
                        }
                    }
                    else //?
                    {
                        string temp = str.Substring(1, str.Length - 1);
                        int countSearch = 0;
                        result.AppendLine(temp);
                        bool flag = false;
                        foreach (string item in dict)
                        {
                            if(countSearch == 20)
                            {
                                break;
                            }
                            if (item.Contains(temp))
                            {
                                int countLetter = 0;
                                for (int i = 0; i < temp.Length; i++)
                                {
                                    if (item[i] == temp[i])
                                    {
                                        countLetter++;
                                    }
                                    else break;
                                }
                                if(countLetter == temp.Length)
                                {
                                    result.AppendLine(item);
                                    countSearch++;
                                    flag = true;
                                }
                                else if(flag == true)
                                {
                                    break;
                                }
                            }
                        }
                        //for (int i = 0; i < dict.Count; i++)
                        //{

                        //    int count = 0;
                        //    bool flagLetter = false;
                        //    for (int j = 0; j < temp.Length; j++)
                        //    {
                        //        if (dict[i][j] == temp[j])
                        //        {
                        //            flagLetter = true;
                        //            count++;
                        //            if (count == temp.Length)
                        //            {
                        //                countSearch++;
                        //                result.AppendLine(dict[i]);
                        //            }
                        //        }
                        //        else break;
                        //        if (countSearch == 20)
                        //        {
                        //            break;
                        //        }
                        //        if(dict[i][j] != temp[j] && flagLetter == true)
                        //        {
                        //            break;
                        //        }
                        //    }
                        //}
                    }
                }
            }
            using(var sw = new StreamWriter("output.txt"))
            {
                sw.Write(result);
            }
        }
    }
}