This problem tests whether you understand BFS beyond the basic template. Standard level-order traversal goes left-to-right every level. Here you alternate directions.
The modification is small (flip a boolean per level) but it proves you can adapt algorithms to new constraints instead of just reciting memorized code. Amazon interviewers use BFS variants like this to distinguish people who understand BFS from those who just copied a template. You'll walk away confident in modifying BFS for any per-level processing rule.