Unflattening a list to sub-objects

edit: I realized it looks like I come for answers right away. I have tried to do it by myself but I came to believe there is a mechanic i do not fully understand. I just can't wrap my head around this problem !

edit2: I misused words ! With "parent" and "children" I do not use the "DOM" meaning ! Here is the HTML used for my current dev

<body>
    <h1>FIRST LEVEL TITLE</h1>
    <h4>test</h4>
    <h2>SECOND LEVEL TITLE</h2>
    <h3>THIRD LEVEL TITLE</h3>
    <h4>test</h4>
    <h3>THIRD LEVEL TITLE</h3>
    <h3>THIRD LEVEL TITLE</h3>
    <h2>SECOND LEVEL TITLE</h2>
    <h3>THIRD LEVEL TITLE</h3>
    <h3>THIRD LEVEL TITLE</h3>
    <h4>test</h4>
    <h4>test</h4>
</body>

The "parent" and "child" hierarchy I want to create is purely virtual (only exists inside my library, not in the HTML) ! My title tags are not nested

Using Bridge.NET and Bridge.JQuery I have successfully retrieved the complete list of title tags from the HTML (h1, h2 and such) and stored them into a flat list. Now I am trying to give this list a hierarchy so every element in my list have a property "Children" containing all the elements that are directly below them, these sub elements containing other sub elements ...

An element is a direct children if there is no intermediate-level element between it and its expected parent.

in the list H2, H3, H4,

H4, is a child of H3, which is a child of H2. But in the list H2, H4, H3,

H3 AND H4 are children of H2

example:

H1 - FIRST LEVEL TITLE
H4 - test
H2 - SECOND LEVEL TITLE
H3 - THIRD LEVEL TITLE
H4 - test
H3 - THIRD LEVEL TITLE
H3 - THIRD LEVEL TITLE
H2 - SECOND LEVEL TITLE
H3 - THIRD LEVEL TITLE
H3 - THIRD LEVEL TITLE
H4 - test
H4 - test

becomes

H1 - FIRST LEVEL TITLE
--->H4 - test
--->H2 - SECOND LEVEL TITLE
--->--->H3 - THIRD LEVEL TITLE
--->--->H4 - test
--->--->--->H3 - THIRD LEVEL TITLE
--->--->--->H3 - THIRD LEVEL TITLE
--->H2 - SECOND LEVEL TITLE
--->--->H3 - THIRD LEVEL TITLE
--->--->H3 - THIRD LEVEL TITLE
--->--->H4 - test
--->--->H4 - test

each of the titles in the flat list are defined as follow (any modification can be made to help achieve what I want)

internal class Title
{
    public TitleLevel Level { get; set; }
    public string Value { get; set; }
    public IEnumerable<Title> Children { get; set; }  

    /* removed irrelevant code */
}

internal enum TitleLevel
{
    H6,
    H5,
    H4,
    H3,
    H2,
    H1,
    DummyValue // exists for technical reasons, I may ask for advice on that later
}

Solution

So here is my solution to what I wanted (un-flatten a list)

The code really isn't pretty, but it works.

It is called like this

var hierarchyList = GetTitleHierarchy(flatList);

And here is the definition

/// <summary>
/// Turns a flat list of titles into a list where each element contains a list of it children, themselves containing a list of childre, ...
/// </summary>
/// <param name="titles">A flat list of titles</param>
/// <returns>A "cascading" list of titles</returns>
internal List<Title> GetTitleHierarchy(List<Title> titles)
{
    return PutInParent(titles);
}

private List<Title> PutInParent(List<Title> titles)
{
    var output = new List<Title>();

    for (int i = 0; i < titles.Count; i++)
    {
        // Copy because if passed by reference we'll get loop-referencing
        var title = titles.Get(i).Copy();

        var childrenCount = CountChildren(titles, i);
        if (childrenCount > 0)
        {
            var subItems = titles.GetRange(i + 1, childrenCount);
            title.Children = PutInParent(subItems);
        }

        output.Add(title);
        i += childrenCount;
    }

    return output;
}

/// <summary>
/// Returns the number of titles after the current index that should be children of the current element
/// </summary>
/// <param name="titles">the flat list of elements</param>
/// <param name="startIndex">the current position in the list</param>
/// <returns></returns>
internal int CountChildren(List<Title> titles, int startIndex)
{
    var clidrenCount = 0;

    foreach (var title in titles.Skip(startIndex + 1))
    {
        if (title.IsLevelLowerThan(titles.Get(startIndex).Level))
            break;

        clidrenCount++;
    }

    return clidrenCount;
}

internal class Title
{
    public TitleLevel Level { get; set; }
    public string Value { get; set; }
    public IEnumerable<Title> Children { get; set; }

    #region Overrides of Object

    public override string ToString()
    {
        return $"{Level} - {Value}";
    }

    #endregion

    public Title Copy()
    {
        return new Title
        {
            Level = Level,
            Value = Value,
            Children = Children.Select(c => c.Copy())
        };
    }

    public bool IsLevelLowerThan(TitleLevel targetLevel)
    {
        return (int) Level <= (int) targetLevel;
    }
}

internal enum TitleLevel
{
    H1 = 1,
    H2 = 2,
    H3 = 3,
    H4 = 4,
    H5 = 5,
    H6 = 6,
    DummyValue = 0
}