I created a video solution to the problem 839C. It is a DFS on trees problem.
The tutorial includes a simple code (no defines, etc) that matches the tutorial and a line-by-line explanation of the code while I write it (helpful for those stuck on the implementation of DFS).
Here is the video solution.
Here is my channel with all the video solutions.
As a side note, I am in the process of adding OI problems (such as USACO and APOI) to Course 3 of VPlanet because I realized that there isn't anywhere to find OI problems separated by topic.