Dic

using System.Collections.Generic;
using System.Text;

namespace TextAnalysis
{
    static class TextGeneratorTask
    {
        public static void NextWord(Dictionary<string, string> dictionary, string startGram, StringBuilder sb)
        {
            foreach (var word in dictionary)
            {
                if (word.Key != startGram) continue;
                sb.Append(" " + word.Value);
                break;
            }
        }

        public static string ContinuePhrase(Dictionary<string, string> nextWords, string phraseBeginning, int wordsCount)
        {
            var str = ""; var flag = true;
            var stringBuilder = new StringBuilder($"{phraseBeginning}");
            var words = phraseBeginning.Split(' ');
            for (var i = 0; i < wordsCount; i++)
            {
                if (words.Length >= 2)
                {
                    str = words[words.Length - 2] + " " + words[words.Length - 1];
                    if (nextWords.ContainsKey(str))
                    {
                        flag = false;
                        NextWord(nextWords, str, stringBuilder);
                    }
                }
                if (flag)
                {
                    str = words[words.Length - 1];
                    NextWord(nextWords, str, stringBuilder);
                }
                words = stringBuilder.ToString().Split(' ');
            }
            phraseBeginning = stringBuilder.ToString();
            return phraseBeginning;
        }
    }
}