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Tesla·Software Engineer·Onsite - Coding / Algorithms·Intermediate

Intermediate
Jul 2026

Summary

Tesla SWE coding round, one graph problem the whole time. Not the hardest interview I've done but I definitely fumbled my way through the BFS part before it clicked.

Questions Asked (1)

Q1

You have an n x n grid of 0s and 1s representing water and land. The grid contains exactly two islands. Find the minimum number of water cells you need to convert to land to connect the two islands.

Algorithms & Data Structures
Author's notes

Took me a minute to even figure out how to separate the two islands first.

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AI HintsAI Generated

Suggested Approach

Model the problem as finding the shortest path between the two islands where the cost of entering a water cell is 1 and land cell is 0. Use multi-source BFS from one island to compute the minimum water cells needed to reach the other island.

Pro tip: Clarify assumptions upfront: whether diagonal moves are allowed and if the grid is guaranteed to have exactly two islands. This shows attention to detail and avoids incorrect solutions.

1. Identify the two islands

Use DFS or BFS to label each island and collect all cells belonging to each. This separates the problem into two distinct regions.

2. Model as shortest path with costs

Treat water cells as cost 1 and land cells as cost 0. The goal is to find the minimum total cost to connect any cell of island A to any cell of island B.

3. Run multi-source BFS from one island

Initialize a queue with all cells of island A and a distance array where land cells have distance 0. Perform BFS, updating distances by adding 1 when moving to a water cell, 0 when moving to land.

4. Find minimum distance to the other island

During BFS, when a cell of island B is reached, the current distance is the minimum water cells needed. Return that distance.

5. Analyze complexity and edge cases

Time complexity is O(n^2) since each cell is processed once. Discuss edge cases like islands already adjacent (answer 0) or grid boundaries.

Key Points to Mention

  • Use BFS with 0-1 weights (deque or two queues) to efficiently handle varying costs.
  • Alternatively, use multi-source BFS from both islands simultaneously to find the shortest meeting point.
  • Explain why DFS alone is insufficient for finding minimum water cells.
  • Discuss time and space complexity: O(n^2) time and O(n^2) space.
  • Mention that the problem is equivalent to finding the shortest path in a grid with weighted cells.
  • Highlight the importance of clarifying movement constraints (4-directional vs 8-directional).

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