Everyone under a manager in an org chart

A company's org chart is a tree: every employee except the top one has one manager. Given the chart and one manager, walk it depth-first and return everyone in that manager's organization, direct reports and everyone below them.

Interface

Unless you agree on a different interface with the interviewer, assume:

• reports: dict[str, list[str]] maps each manager to their direct reports, in a fixed order; people with no reports may be missing from it.
• everyone_under(reports, manager: str) -> list[str] — the manager's whole organization in depth-first order (a report, then that report's organization, then the next report), not including the manager.

Example

reports = {"ana": ["ben", "cy"], "ben": ["dee", "eli"], "cy": ["fay"], "eli": ["gus"]}

everyone_under(reports, "ana")  ->  ["ben", "dee", "eli", "gus", "cy", "fay"]
everyone_under(reports, "ben")  ->  ["dee", "eli", "gus"]
everyone_under(reports, "fay")  ->  []

Constraints

• Up to 10^5 employees; the chart can be deep (a long chain of single reports).

Clarifying questions

• Is the input a map of direct reports, or a list of (employee, manager) pairs I must turn into one?
• Should the manager be in the result?
• Can the data contain a cycle or someone with two managers?

What a strong answer covers

• Recursive depth-first search, then an iterative version with an explicit stack that pushes reports in reverse so the order matches.
• O(size of the organization) time and O(depth) space; why recursion can overflow on a deep chain.
• Building the reports map from (employee, manager) pairs when that is the input.
• A visited set if the data might contain a cycle.

Follow-ups

• The size of every manager's organization in one pass (post-order).
• Print the chart indented by level.
• Find the lowest common manager of two employees.