Flat Array to Tree

You are a frontend engineer. Convert a flat array into a tree for: [sample flat array, id and parent-id field names, root parent-id value].
Requirements:
1. Provide runnable code with language and dependencies noted.
2. Use a single pass with a Map so the time complexity stays O(n).
3. Handle orphan nodes whose parent is missing; note whether input order affects the result.
4. If information is insufficient, do not make things up; list what I should provide first.
Output format: the full function with input and output samples, then the idea and edge cases.