Binary Tree Level Order Traversal
Reported by candidates from Bloomberg's online assessment. Pattern, common pitfall, and the honest play if you blank under the timer.
The mistake that sinks a first attempt at Bloomberg's Binary Tree Level Order Traversal is mixing up levels while you walk the tree. This one was reported in February 2021, and it's a clean breadth-first search problem on a tree. You return the values level by level, top to bottom, left to right within each level. The example [3,9,20,null,null,15,7] gives [[3],[9,20],[15,7]]. It looks easy, and it is, until you flatten everything into one list and lose the level boundaries. If you blank during the live OA, StealthCoder is the invisible safety net that reads the problem and hands you the pattern.
The problem
Return the tree values level by level from top to bottom. Within each level, list nodes from left to right. Function levelOrder(root: TreeNode) → int[][] Examples Example 1 root = [3,9,20,null,null,15,7] return = [[3],[9,20],[15,7]] The root forms level zero, followed by its children and then the final two grandchildren. Constraints The tree contains between 0 and 10000 nodes. Node values fit in a signed 32-bit integer.
Reported by candidates. Source: FastPrep
Pattern and pitfall
The trick is a queue plus a level-size snapshot. Push the root. While the queue isn't empty, record its current length n, then pop exactly n nodes. Those n nodes are one level. Append their values as one inner list, and push their children for the next round. That snapshot is the whole problem. The common pitfall is reading queue length inside the loop condition, so children from the next level bleed into the current one. Also handle the empty tree first: the constraints allow 0 nodes, so return [] when root is null. Don't forget left child goes before right child. A recursive DFS with a depth parameter also works, appending to result[depth], but BFS is cleaner to explain. Complexity is O(n) time and O(n) space. If the live OA freezes you, StealthCoder can supply this skeleton so you only have to type it.
Memorize the pattern. If you can't, run StealthCoder. The proctor sees the IDE. They don't see what's behind it.
You can drill Binary Tree Level Order Traversal 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. Made by an engineer who treats the OA as theater. If yours is tonight, you don't have time to grind. You have time to hedge.
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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Make sure you actually pass Bloomberg's OA.
Bloomberg reuses patterns across OAs. Made by an engineer who treats the OA as theater. If yours is tonight, you don't have time to grind. You have time to hedge. Works on HackerRank, CodeSignal, CoderPad, and Karat.
Binary Tree Level Order Traversal FAQ
How hard is Binary Tree Level Order Traversal really?+
It's easy. It's the standard BFS-on-a-tree template. The only real risk is sloppy level grouping or forgetting the empty tree. If you've written a queue-based BFS once, you can finish this in a few minutes.
What's the trick to keeping levels separate?+
Snapshot the queue size at the start of each round. Pop exactly that many nodes, collect their values into one list, and enqueue their children. Children go to the next round because you already fixed the count before adding them.
Can I solve it with DFS instead of BFS?+
Yes. Pass a depth argument, and when you reach a new depth, append a fresh list to the result. Then push the node's value into result[depth]. Visit left before right so order within each level stays correct. BFS is still simpler to explain.
What edge cases should I test?+
Test a null root, which should return an empty list. Test a single node, a skewed tree where every level has one node, and a full tree. The constraints allow up to 10000 nodes, so a skewed tree makes recursion deep, which favors the iterative queue.
How do I prepare for this in 48 hours?+
Write the BFS template from memory three times, then do the variants: zigzag order, right side view, and bottom-up order. They all reuse the same level-size loop. Spend the rest of your time on other tree and queue patterns rather than re-reading this one.