How to exchange string content in a text file in C#?

Question

I have a text file as follows:

1 ... 3 4 2    
2 ... 3 21 4  
3 ... 6 4 21 15  
4 ... 14 21 12   

I want to edit these strings, so that numbers after dotted parts to be splitted corresponding to the first number of each string. For example,

1  
2 1  
3 1 2  
4 1 2 3  
...  
21 3 4  

How can I do this?
Note: I obtain the first number group from a text file and edit it string by string. After that, I have written edited strings to the text file. In light of this, sample part of my code to obtain the first number group is provided as below:

        for (var i = 0; i < existingLines.Length; i++)
        {
            var split = existingLines[i].Split('\t');
            var act = i - 1;
            var sc1 = int.Parse(split[6]);
            var sc2 = int.Parse(split[7]);
            appendedLines.Add(string.Format("{0} {1} {2}", act, sc1, sc2));
        }  

Answer 1

This LINQ code should get you started

        string path = "c:\\temp\\test.txt";

        using (var sr = new StreamReader(path))
        {
            var lines = new List<IEnumerable<int>>();

            while (!sr.EndOfStream)
            {
                lines.Add(sr.ReadLine().Split(new[] { '.', ' ' }, StringSplitOptions.RemoveEmptyEntries).Select(x => int.Parse(x)));
            }

            foreach (var node in lines.SelectMany(x => x).Distinct().OrderBy(x => x))
            {
                var predecessors = lines.Where(x => x.Skip(1).Contains(node))
                    .Select(x => x.First())
                    .OrderBy(x => x);

                Console.WriteLine(node + " " + string.Join(" ", predecessors));
            }
        }

Output

2 1
3 1 2
4 1 2 3
6 3
12 4
14 4
15 3
21 2 3 4