Frontend: Event Delegation (closest() Walk)

MediumTypeScript

Description

> This problem is paper-only - see explanation for the canonical JS solution. Instead of attaching N click handlers to N buttons, attach one to a common ancestor and use `event.target.closest(selector)` to identify the actual clicked element. The auto-grader validates the closest() walk. Given a flat parent->child tree (`{ id, parentId | null, classes: string[] }` rows) and a starting node id, walk up via parent links and return the **first ancestor (or self) whose classes contain the target class**, or `null` if none. **For the auto-grader:** implement `findAncestor(nodes, startId, targetClass)`. Return the matching node id, or null.

Example 1

Input: nodes = [{id:"a",parentId:null,classes:["root"]},{id:"b",parentId:"a",classes:["item"]}], startId = "b", targetClass = "root"

Output: "a"

Example 2

Input: nodes = [{id:"a",parentId:null,classes:["root"]},{id:"b",parentId:"a",classes:["item","row"]}], startId = "b", targetClass = "row"

Output: "b"

Explanation: Self matches.

Example 3

Input: nodes = [{id:"a",parentId:null,classes:["x"]}], startId = "a", targetClass = "missing"

Output: null

Constraints

- 0 <= nodes.length <= 1000 - node ids are unique strings. - parentId references an existing node or is null. - Tree is acyclic (you can assume).

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