Coding (Algo)

Lowest common ancestor: trace parent relationships and find the deepest shared ancestor

Lowest common ancestor: Given a heap-indexed binary tree with unique non-null values and two values p and q, return their lowest common ancestor value, or null when either value is absent.

Input: A JSON object { "tree": (number|null)[], "p": number, "q": number }.

Output: A JSON number or null.

Constraints:

0 <= tree.length <= 100000
Non-null tree values are unique
A node may be its own ancestor

Read one JSON value from stdin and print exactly one JSON-serialized answer to stdout with no extra text.

Difficulty: HARD. Variant: 9. Balance correctness, maintainability, and clear review-ready code.

Practice context

This coding (algo) exercise trains prompt interpretation, explicit assumptions, a concrete response, and a concise explanation of the decisions made. It is marked hard difficulty.

The signed-in workspace provides the tools required by the round, such as recording,

What this task practices

This coding (algo) exercise trains the same sequence expected in an interview: understand the prompt, make assumptions explicit, produce a concrete answer, and explain the decisions behind it. The task is marked hard difficulty. The private workspace adds the tools required by this round, such as recording, code execution, diagrams, evidence panels, or structured notes. It also preserves the attempt so later feedback can be compared with previous work. Evaluation is based on task-specific criteria and the candidate seniority selected in the preparation path. Public pages never expose the reference solution, hidden tests, evaluator instructions, or another candidate’s work.