Bytedance·Software Engineer·Technical Phone Screen·Intermediate

IntermediatePrefer not to say

Jun 2026

Summary

Bytedance SWE interview that threw a curveball on a classic tree problem. Not the worst experience but the follow-up variant is what gets you.

Questions Asked (1)

Given a binary tree and two nodes p and q, find their lowest common ancestor. Then: what if q might not exist in the tree at all?

Algorithms & Data Structures

Author's notes

The base LCA problem I had down cold.

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AI HintsAI Generated

Suggested Approach

Start by explaining the classic recursive LCA algorithm for a binary tree, which returns the node if it matches p or q, and otherwise recurses on both subtrees. Then address the follow-up by modifying the recursion to track whether both nodes were actually found, returning a sentinel or null if q is missing.

Pro tip: Explicitly discuss the trade-off between the simpler two-pass approach (first check existence, then find LCA) and the more efficient one-pass method that returns a status flag, showing you understand both correctness and performance.

1. Clarify assumptions and constraints

Ask whether the tree is binary (not BST), whether nodes have parent pointers, and whether p and q are guaranteed to be distinct and non-null. Confirm that the follow-up assumes q may be absent.

2. Present the classic LCA algorithm

Describe the recursive function that returns the current node if it equals p or q, otherwise recurses left and right. If both recursive calls return non-null, the current node is the LCA; otherwise return the non-null result.

3. Analyze time and space complexity

State that the algorithm visits each node once, giving O(n) time, and uses O(h) space for the recursion stack, where h is the tree height (O(n) worst case for a skewed tree).

4. Address the follow-up: q might not exist

Explain that the classic algorithm can incorrectly return p as the LCA if q is missing. To fix this, modify the recursion to return a pair (node, foundCount) or use a wrapper that tracks whether both p and q were encountered.

5. Implement and test the modified approach

Show how to propagate a boolean flag or use a helper that returns null if either node is not found. Walk through edge cases: q missing, p missing, both missing, and one node being an ancestor of the other.

Key Points to Mention

The classic LCA algorithm assumes both nodes exist; it fails when one is missing because it may return the existing node prematurely.
To handle missing nodes, track found status for both p and q during recursion, e.g., by returning a pair (node, foundCount) or using a global counter.
Time complexity remains O(n) and space O(h) for the one-pass modified approach.
An alternative is a two-pass approach: first traverse to check if both nodes exist, then run classic LCA. This is O(n) time but two passes.
Edge cases: p or q is the root, p is an ancestor of q, q is missing, both missing, and duplicate values in the tree (if nodes are identified by value).
If parent pointers are available, you can find LCA by finding intersection of paths to root, but this uses extra space.

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