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Bytedance·Software Engineer·Technical Phone Screen·Intermediate

Intermediate
May 2026

Summary

Bytedance software engineer interview with a classic grid traversal problem. Nothing too surprising but the pressure of doing it live is a different beast than grinding it at home.

Questions Asked (1)

Q1

Given a 2D binary grid representing land ('1') and water ('0'), count the number of islands, where an island is a group of horizontally or vertically connected land cells.

Algorithms & Data Structures
Author's notes

Classic BFS/DFS problem and I still fumbled the edge case handling for a second.

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AI HintsAI Generated

Suggested Approach

Treat the grid as a graph and use DFS/BFS to explore each island, marking visited cells to avoid recounting. Iterate through each cell; when you find an unvisited '1', increment the island count and flood-fill to mark all connected land.

Pro tip: Clarify edge cases upfront (empty grid, all water, all land) and mention that you can optimize space by modifying the grid in-place if allowed, or use a separate visited matrix if not.

1. Clarify and validate

Confirm grid dimensions, connectivity definition (4-directional), and whether input can be modified. Discuss edge cases like empty grid or no land.

2. Choose traversal method

Decide between DFS (recursive or iterative) and BFS. Consider trade-offs: DFS is simpler but may risk stack overflow on large grids; BFS uses a queue and avoids recursion depth issues.

3. Implement traversal

Write a helper function that, given a starting cell, marks all connected land cells as visited. Use a visited set/matrix or modify the grid in-place (e.g., change '1' to '0').

4. Count islands

Iterate through every cell in the grid. When encountering an unvisited '1', increment the island count and invoke the traversal to mark the entire island.

5. Analyze complexity

State time complexity O(M×N) since each cell is visited once, and space complexity O(M×N) in worst case for recursion/queue or visited matrix.

Key Points to Mention

  • Graph traversal (DFS/BFS) on a 2D grid
  • Visited tracking to avoid infinite loops and double-counting
  • In-place modification vs. auxiliary visited matrix
  • Time and space complexity analysis
  • Handling edge cases (empty grid, single row/column, all land/water)
  • Iterative vs. recursive DFS and potential stack overflow

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