Bloomberg·Software Engineer·Technical Phone Screen·Intermediate

IntermediatePrefer not to say

Jun 2026

Summary

Bloomberg coding round, one problem but with a twist baked in from the start. Pretty clean interview overall, just needed to think past the standard version of the problem.

Questions Asked (1)

Given an m x n grid of land and water cells, find all islands (4-connected groups of land). Then, given an array of target sizes, return an array where each entry is the count of islands whose size exactly matches the corresponding target.

Algorithms & Data Structures

Author's notes

It's Number of Islands but you can't just count them, you need to track the size of each one.

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AI HintsAI Generated

Suggested Approach

Use BFS or DFS to traverse each unvisited land cell, marking visited cells and counting the size of each island. Store island sizes in a frequency map, then for each target size, look up its count in O(1) time. This yields O(m*n + t) time and O(m*n) space.

Pro tip: Mention that you can avoid modifying the input grid by using a separate visited matrix or by temporarily mutating the grid and restoring it, and discuss trade-offs. Also, note that if the grid is very large, a union-find approach can be more cache-friendly and parallelizable.

1. Clarify and Plan

Confirm the definition of 4-connectivity, input constraints (e.g., grid size, target array size), and expected output format. Decide on traversal method (BFS/DFS) and data structures.

2. Traverse and Count Islands

Iterate through each cell; when encountering unvisited land, perform BFS/DFS to explore the entire island, marking cells as visited and counting its size.

3. Record Island Sizes

Store each island's size in a hash map (size -> frequency) or a list if sizes are bounded. This allows O(1) lookup for target sizes.

4. Answer Queries

For each target size, retrieve its frequency from the map (or 0 if absent) and append to the result array.

5. Analyze Complexity

State time complexity O(m*n + t) and space complexity O(m*n) for visited tracking, and discuss potential optimizations like union-find or early termination.

Key Points to Mention

4-connectivity definition and how to handle boundaries
Choice of BFS vs DFS and trade-offs (stack depth, queue memory)
Using a visited matrix or in-place marking to avoid revisiting cells
Storing island sizes in a frequency map for O(1) target lookups
Time and space complexity analysis
Edge cases: empty grid, no land, target sizes larger than grid, duplicate targets

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