using System;

using System.Collections.Generic;

using System.Text;

public class Program

{

    public static LinkedList<WordPosition> list = new LinkedList<WordPosition>();

    public static void Main()

    {

        string sentence = "this is a test";

        addWordsToStructure(sentence);

        LinkedListNode<WordPosition> root = list.First;

        while(root != null)

        {

            Console.WriteLine(root.Value.word + " " + root.Value.position); 

            root = root.Next; 

        }

    }

    public static void addWordsToStructure(string sentence)

    {

        int indexStart = 0;

        int indexEnd = 0;

        int position = 1;

        for(int i = 0; i < sentence.Length; i++)

        {

            if(sentence[i] == ' ')

            {

                indexEnd = i - 1;

                string word = extractWord(sentence, indexStart, indexEnd);

                addWordToList(word, position);

                indexStart = i + 1;

                position++;

            }

            if(i == sentence.Length - 1)

            {

                indexEnd = i;

                string word = extractWord(sentence, indexStart, indexEnd);

                addWordToList(word, position);

                position++;

            }

        }

    }

    public static string extractWord(string sentence, int indexStart, int indexEnd)

    {

        StringBuilder word = new StringBuilder();

        for(int i = indexStart; i <= indexEnd; i++)

        {

            word.Append(sentence[i]);

        }

        return word.ToString();

    }

    public static void addWordToList(string word, int position)

    {

        WordPosition wordPos = new WordPosition(){

            word = word,

            position = position,

        };

​

        list.AddLast(wordPos);

    }

    public class WordPosition

    {

        public string word;

        public int position;

    }

}