Filter Undesired Comments and Their Descendants

Comments are numbered by array index. parent[i] is -1 for a root or the ID of comment i's parent. animal[i] is cat, dog, or neutral.
A user whose preference is cat considers dog comments undesired, and vice versa. Return every undesired comment plus every descendant of any undesired comment, sorted by ID. Neutral comments are included only when they descend from a marked comment.

Function
undesiredCommentIds(parent: int[], animal: String[], preference: String) → int[]

Examples
Example 1
parent = [-1,0,0,1,1]
animal = ["neutral","dog","cat","cat","neutral"]
preference = "cat"
return = [1,3,4]
Comment 1 is undesired, so both of its descendants are included regardless of label.
Example 2
parent = [-1,0,0]
animal = ["dog","cat","neutral"]
preference = "dog"
return = [1]
Only the cat-labeled child is undesired.

Constraints
0 <= parent.length <= 100000; both arrays have equal length.
The parent links form a forest.
preference is cat or dog.

Build children lists from the parent array, since parent[i] points up and you need to go down. Mark every comment whose animal is the opposite of the preference as a seed. Then run a BFS or iterative DFS from each seed, marking all descendants regardless of label. Collect marked IDs and return them sorted. A simpler trick: since you can walk up, you can also check for each node whether any ancestor is undesired, memoizing results. The pitfall is recursion depth. A forest with 100000 nodes can be a single chain, so recursive DFS can overflow the stack in some languages. Use an explicit stack or queue. Another pitfall is including neutral comments that aren't under a seed. Neutral alone never qualifies. Scanning IDs in order at the end gives sorted output for free, no sort needed. If the live problem trips you up, StealthCoder is the hedge that reads the screen and hands you working code.

Memorize the pattern. If you can't, run StealthCoder. The proctor sees the IDE. They don't see what's behind it.