Count Levels
Reported by candidates from Blackrock's online assessment. Pattern, common pitfall, and the honest play if you blank under the timer.
Blackrock's July OA included a tree traversal problem called Count Levels. You're likely given a tree structure and need to count or return information about each level. This isn't about finding the tallest node or summing values. The trick is recognizing that level-order traversal (BFS) is the natural fit here, not DFS. If you blank on the live OA, StealthCoder will read the exact problem and surface the pattern in seconds so you can code with confidence.
Pattern and pitfall
Count Levels typically means returning the number of levels in a tree, or building a result grouped by level. The pattern is breadth-first search (BFS) using a queue. You process all nodes at depth d before moving to depth d+1. A common pitfall is using DFS and manually tracking depth, which works but is slower and messier to code under pressure. The clean approach: initialize a queue with the root, then while the queue is not empty, drain it level by level, increment a counter, and enqueue children. If the problem asks for values per level, store them in a list of lists. StealthCoder acts as your safety net if you forget the BFS skeleton during the live assessment.
If you see this problem in your OA tomorrow, the play is to recognize the pattern in 30 seconds. StealthCoder buys you that recognition.
You can drill Count Levels cold, or you can hedge it. StealthCoder runs invisibly during screen share and surfaces a working solution in under 2 seconds. The proctor sees the IDE. They don't see what's behind it. Built by an Amazon engineer who passed his OA cold and still thinks the filter is broken.
Get StealthCoderRelated leaked OAs
This OA pattern shows up on LeetCode as binary tree level order traversal. If you have time before the OA, drill that.
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Count Levels FAQ
Is this definitely a tree problem or could it be a graph?+
If the problem says 'tree' or 'binary tree' explicitly, it's acyclic and you don't need a visited set. If it's a graph, you'll see cycles or undirected edges. Count Levels at Blackrock in July was tree-based. Assume tree unless told otherwise.
Do I need to return the count or the actual levels?+
That depends on the exact wording you'll see in the OA. If it asks for 'the number of levels', return an integer. If it asks 'return a list of lists, one per level', return a 2D structure. Read the output format carefully before coding.
Is DFS acceptable or is BFS required?+
BFS is cleaner and more intuitive for this pattern. DFS requires tracking depth explicitly, which adds mental load during a timed OA. BFS naturally processes one level at a time. Stick with BFS unless you're very confident with recursive DFS depth tracking.
What if the tree is empty or has only a root?+
An empty tree has 0 levels. A tree with only a root has 1 level. Handle these edge cases by checking if the root is null at the start, and initializing your level counter or queue carefully. These are common test cases.
How long should this take to code in an interview?+
BFS with level counting should take 5-8 minutes to write, test, and debug. The algorithm is straightforward. If you're spending 20+ minutes, you're overthinking. Use StealthCoder on the live OA to confirm the pattern match and unblock yourself fast.