void Main()
{
    var inputTree = new Tree<char>('a',
        new Tree<char>('b',
            null,
            new Tree<char>('c',
                null,
                null)
            ),
        new Tree<char>('d',
            null,
            null)
    );

    var outputTree = Rebuild(inputTree);

    outputTree.Dump();

    Debug.Assert(outputTree.Value == 1, "1");
    Debug.Assert(outputTree.Left.Value == 2, "2");
    Debug.Assert(outputTree.Right.Value == 3, "3");
    Debug.Assert(outputTree.Left.Right.Value == 4, "4");

    Debug.Assert(outputTree.Left.Left == null, "a");
    Debug.Assert(outputTree.Left.Right.Left == null, "b");
    Debug.Assert(outputTree.Left.Right.Right == null, "c");
    Debug.Assert(outputTree.Right.Left == null, "d");
    Debug.Assert(outputTree.Right.Right == null, "e");
}

Tree<int> Rebuild<T>(Tree<T> inputTree)
{
    return InternalRebuild(1, null, inputTree);
}

Tree<int> InternalRebuild<T>(int? value, int? sibling, Tree<T> inputTree)
{
    if (inputTree == null)
        return null;

    var max = Math.Max(value.Value, sibling.GetValueOrDefault());
    var lv = inputTree.Left == null ? (int?) null : max + 1;
    var rv = inputTree.Right == null ? (int?) null : max + (lv == null ? 1 : 2);

    return new Tree<int>(value.Value,
        InternalRebuild(lv, rv, inputTree.Left),
        InternalRebuild(rv, lv, inputTree.Right));
}

// Define other methods and classes here
public class Tree<T>
{
    public T Value { get; private set; }
    public Tree<T> Left { get; private set; }
    public Tree<T> Right { get; private set; }

    public Tree(T value, Tree<T> left, Tree<T> right)
    {
        Value = value;
        Left = left;
        Right = right;
    }
}