For source code, build instructions, documentation. Open source and commercial friendly. Pothos is a completely open source framework from the API, to the GUI, to the available toolkits. Pothos is freely available under a permissive license to be used in a variety of open source projects, and for private or commercial uses. Read more about the available features on the Pothos features summary page.

This problem is famous due to the wellknown twet. T, by Max Howell. I can not say it is hard or easy. Here just provide the iterative method as recursive method is really too easy. In general, you just need to swap its left and right for each node. It is natural to think it as a pre-order problem.