← Scale AI Interview Insights

Scale AI·Software Engineer·Technical Phone Screen·Intermediate

Intermediate
May 2026

Summary

Scale AI software engineer interview with a tree algorithm problem. Single coding round focused on graph traversal fundamentals, nothing too wild but the N-ary twist on a classic problem kept me on my toes.

Questions Asked (1)

Q1

Given the root of an N-ary tree and two distinct node references p and q, find their lowest common ancestor using a recursive DFS approach that completes in a single traversal.

Algorithms & Data Structures
Author's notes

I know LCA for binary trees pretty well but the N-ary version tripped me up for a minute because you can't just go left/right anymore.

Create a free account to read the full note

AI HintsAI Generated

Suggested Approach

Use a post-order DFS that returns the current node if it matches p or q, otherwise recursively searches children. If two children return non-null, the current node is the LCA; otherwise, propagate the non-null result upward. This single traversal finds the LCA in O(N) time and O(H) space.

Pro tip: Clarify that the algorithm assumes both p and q exist in the tree; if not, you may need a separate check or a modified return value. Mentioning this edge case shows attention to detail and avoids incorrect assumptions.

1. Define the recursive function

Write a function that takes a node and returns the LCA if found, or the node itself if it matches p or q, or null otherwise.

2. Handle base cases

If the current node is null, or equals p or q, return the current node immediately.

3. Recurse on children

For each child, call the function and collect the results. Count how many non-null results are returned.

4. Determine LCA at current node

If two or more children return non-null, the current node is the LCA. If exactly one child returns non-null, return that result. If none, return null.

5. Analyze complexity

Explain that each node is visited once, giving O(N) time, and recursion depth is O(H) where H is tree height, so O(H) space.

Key Points to Mention

  • Single traversal post-order DFS
  • Base case: node equals p or q
  • Counting non-null returns from children
  • LCA when two children return non-null
  • Time complexity O(N), space O(H)
  • Assumption that both p and q exist in the tree

AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.