Apple·Software Engineer·Technical Phone Screen·Intermediate

Intermediate

Apr 2026

Summary

Apple coding interview, one algorithmic problem that looked manageable until I actually had to implement it under pressure. The question was graph-heavy and required chaining two different BFS passes together, which is the kind of thing that sounds obvious in retrospect.

Questions Asked (1)

Given an n x n binary grid where 1s represent land and 0s represent water, find the shortest path of water cells (0s) you'd need to convert to land in order to connect any two distinct islands. Islands are groups of 1s connected in four directions.

Algorithms & Data Structures

Author's notes

I knew the general shape of the solution pretty quickly: find the islands first, then BFS outward from one to reach the other.

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AI HintsAI Generated

Suggested Approach

Use multi-source BFS from all islands simultaneously to compute the minimum distance from each water cell to the nearest island. Then, for each water cell, consider connecting two different islands via that cell, and take the minimum over all such cells. Alternatively, use BFS from each island and track the minimum sum of distances from two different islands.

Pro tip: Clarify that the problem assumes at least two islands exist; if not, return 0 or handle appropriately. Also, mention that the solution can be optimized by early termination when the minimum possible distance is found.

1. Identify and label islands

Traverse the grid to find all islands and assign each a unique label using BFS/DFS. Store the cells of each island.

2. Multi-source BFS from all islands

Initialize a queue with all land cells, each tagged with its island label. Perform BFS to compute the distance from each water cell to the nearest island, and record which island that is.

3. Find minimum connection cost

For each water cell, if it is adjacent to two different islands or if during BFS two frontiers from different islands meet, compute the sum of distances and update the minimum.

4. Handle edge cases and return result

If there are fewer than two islands, return 0. Otherwise, return the minimum number of water cells to convert.

Key Points to Mention

Use BFS for shortest path in unweighted grid.
Multi-source BFS efficiently computes distances from all islands simultaneously.
Track island labels to ensure connecting distinct islands.
Consider both direct adjacency and paths through water cells.
Time complexity: O(n^2) as each cell is processed a constant number of times.
Space complexity: O(n^2) for the queue and distance arrays.

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