← Meta Interview Insights

Meta·Machine Learning Engineer·Technical Phone Screen·Senior

Senior
May 2026

Summary

Meta ML engineer interview with a grid-based coding problem. Not a lot of context in what I remember but the problem itself was meaty enough to keep me busy.

Questions Asked (1)

Q1

Given a 2D grid of 1s and 0s representing land and water, find the largest perimeter among all islands in the grid.

Algorithms & Data Structures
Author's notes

Classic grid traversal but the perimeter part tripped me up a bit.

Create a free account to read the full note

AI HintsAI Generated

Suggested Approach

Clarify the problem constraints and edge cases, then propose a solution using DFS/BFS to traverse each island while computing its perimeter by counting water-adjacent edges. Discuss time and space complexity, and consider optimizations like iterative traversal to avoid recursion limits.

Pro tip: Mention that you can compute the perimeter during traversal by adding 4 for each land cell and subtracting 2 for each shared edge with another land cell, which simplifies the calculation and avoids separate boundary checks.

1. Clarify the problem

Ask about grid size, whether islands are connected 4-directionally, and if the grid can be empty. Confirm that perimeter is the total number of edges between land and water or grid boundary.

2. Choose traversal method

Select DFS or BFS to explore each island. Mention that BFS with a queue avoids recursion depth issues for large grids.

3. Compute perimeter during traversal

For each land cell, add 4 to the perimeter, then subtract 2 for each adjacent land cell already visited or to be visited, ensuring each shared edge is subtracted once.

4. Track maximum perimeter

Maintain a variable to store the maximum perimeter found so far. After traversing an island, update the maximum if its perimeter is larger.

5. Analyze complexity and edge cases

State that time complexity is O(m*n) and space complexity is O(m*n) in worst case. Discuss edge cases like no islands, single cell, and large grids.

Key Points to Mention

  • Use DFS or BFS to traverse each island, marking visited cells to avoid revisiting.
  • Compute perimeter by counting edges adjacent to water or grid boundary, or by the formula: perimeter = 4 * land_cells - 2 * shared_edges.
  • Time complexity is O(m*n) where m and n are grid dimensions, as each cell is visited once.
  • Space complexity is O(m*n) for the visited set or recursion stack in worst case.
  • Handle edge cases: empty grid, no land, single land cell, and islands touching the grid boundary.
  • Optimize by using an iterative BFS to avoid stack overflow for large islands.

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