textAnalysis

using System.Collections.Generic;

namespace TextAnalysis
{
    internal static class FrequencyAnalysisTask
    {
        public static Dictionary<string, string> GetMostFrequentNextWords(List<List<string>> text)
        {
            var result = new Dictionary<string, string>();
            var frequencies = BuildFrequencies(text);

            foreach (var bigram in frequencies)
                result.Add(bigram.Key, GetMostFrequent(bigram.Value));

            return result;
        }

        private static Dictionary<string, Dictionary<string, int>> BuildFrequencies(List<List<string>> text)
        {
            var frequencies = new Dictionary<string, Dictionary<string, int>>();

            foreach (var sentence in text)
            {
                for (var i = 0; i < sentence.Count - 1; i++)
                {
                    IncrementFrequencySafely(frequencies, sentence[i], sentence[i + 1]);
                    if (i < sentence.Count - 2)
                        IncrementFrequencySafely(frequencies, sentence[i] + " " + sentence[i + 1], sentence[i + 2]);
                }
            }

            return frequencies;
        }

        private static void IncrementFrequencySafely(Dictionary<string, Dictionary<string, int>> frequencies,
            string beginning, string nextWord)
        {
            if (!frequencies.ContainsKey(beginning))
                frequencies[beginning] = new Dictionary<string, int>();

            if (!frequencies[beginning].ContainsKey(nextWord))
                frequencies[beginning][nextWord] = 0;
            frequencies[beginning][nextWord]++;
        }

        private static string GetMostFrequent(Dictionary<string, int> wordFrequencies)
        {
            var result = "";
            var maxFrequency = -1;

            foreach (var word in wordFrequencies)
                if (word.Value >= maxFrequency && string.CompareOrdinal(word.Key, result) < 0)
                {
                    result = word.Key;
                    maxFrequency = word.Value;
                }

            return result;
        }
    }
}