Bloomberg·Software Engineer·Technical Phone Screen·Intermediate

IntermediatePrefer not to say

Jun 2026

Summary

Bloomberg SWE coding round, one problem, stack-based string/array manipulation. The problem looked like a LeetCode variant I'd seen before and that familiarity almost worked against me.

Questions Asked (1)

Given a 1D array (or string) of elements, repeatedly remove every maximal consecutive run of three or more identical items. After each removal, the remaining elements close the gap and new runs may form, which should also be removed. Continue until no such run exists. Return the final array. Solve this efficiently using a stack of (value, count) pairs.

Algorithms & Data Structures

Author's notes

I recognized the Candy Crush problem immediately and that was almost my downfall.

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AI HintsAI Generated

Suggested Approach

Use a stack of (value, count) pairs to process the array in a single pass. For each element, if it matches the top of the stack, increment the count; otherwise push a new pair. If the count reaches 3, pop the pair, which naturally handles cascading removals. Finally, reconstruct the array from the stack.

Pro tip: Emphasize that this stack-based solution is O(n) time and O(n) space, which is optimal. Mention that a naive approach with repeated scans would be O(n^2) and inefficient for large inputs, showing you consider scalability.

1. Understand the problem and edge cases

Clarify that removals cascade and that the array can be empty. Consider edge cases like all elements identical, no runs of three, or multiple cascading removals.

2. Choose the stack data structure

Use a stack where each element is a pair (value, count). This efficiently tracks consecutive runs and allows O(1) updates and removals.

3. Process each element

Iterate through the input. If the current element matches the top value, increment its count; otherwise push (value, 1). If count reaches 3, pop the pair.

4. Reconstruct the result

After processing, pop all pairs and expand them back into an array (or string) in the correct order. Return the result.

5. Analyze complexity and test

State that time and space are O(n). Walk through a small example to verify correctness, including a case with cascading removals.

Key Points to Mention

Stack of (value, count) pairs to track consecutive runs
Single pass through the input for O(n) time complexity
Cascading removals handled automatically by popping when count reaches 3
Reconstruction of the final array from the stack
Edge cases: empty input, no removals, all elements removed
Comparison with naive O(n^2) approach to highlight efficiency

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