LeetCode, Hard++ (Acceptance 24%, Latest): 2867. Count Valid Paths in a Tree. DFS. O(n). Swift

There are tons of similar poorly explained solutions for various challenges on LeetCode. Why should this one be on Habr? A Habr article requires more details, such as:

• Images to visualize the challenge and the algorithm.

• Descriptions for every algorithm step. For example, the code for finding primes doesn't even mention Sieve of Eratosthenes.

• Why the complexity is what it is.

• No unclear abbreviations in the code.

• Alternative approaches.

I solved this challenge today during the contest, but I can't understand how your code works.