Google Round 1 Coding: Independent Level Iterators Over a Shared Binary Tree

Role: Software Engineer
Result: Passed the coding round
Report time: November 2025

The source title calls this an internship interview, while the structured fields classify it as full-time New Grad. The interview details below are preserved without resolving that metadata conflict.

Coding: streaming level-order iterator

Implement an iterator that returns a binary tree one level at a time without traversing and storing the entire tree in advance.

The candidate kept a queue containing the current level. Each next() call returned that level and prepared the children for the following call.

The design uses memory proportional to the current level width. The candidate contrasted it with performing a complete BFS and storing every level before iteration, which would use unnecessary memory.

Follow-up: multiple iterators

Several iterators may read the same tree concurrently. Each iterator should maintain its own queue and traversal position while the underlying tree remains shared and read-only. If the environment permits mutation, the candidate discussed lightweight locking or version snapshots.

Practice: Independent Level Iterators Over a Shared Binary Tree

A commenter asked whether a Python solution needed to implement both __iter__ and __next__, but no author answer was preserved.