Molocoads·Software Engineer·Technical Phone Screen·Intermediate

IntermediatePrefer not to say

Jun 2026Remote

Summary

Molocoads coding round, one problem the whole time. Grid simulation with bubble elimination and gravity. Nothing flashy, just had to get the mechanics right without tripping over edge cases.

Questions Asked (1)

Given a 2D grid of colored bubbles, identify and eliminate groups of adjacent same-color cells that meet a size threshold, then apply gravity so remaining cells fall down and empty cells rise to the top. Return the resulting grid.

Algorithms & Data Structures

Author's notes

The problem itself isn't hard conceptually but I kept second-guessing my BFS for the grouping step.

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AI HintsAI Generated

Suggested Approach

Clarify the problem constraints and edge cases, then propose a solution that identifies groups via BFS/DFS, marks them for removal, and applies gravity column-wise. Discuss time/space complexity and potential optimizations.

Pro tip: Mention that you would handle large grids by using an in-place marking technique to avoid extra space, and discuss how to optimize gravity using two pointers per column.

1. Clarify Requirements

Ask about grid dimensions, color representation, size threshold, and whether elimination cascades. Confirm input/output format and constraints.

2. Identify Groups

Use BFS/DFS to find all connected components of same-colored cells. Track visited cells to avoid reprocessing.

3. Eliminate Groups

For each group meeting the size threshold, mark cells as empty (e.g., set to null or a special value).

4. Apply Gravity

For each column, shift non-empty cells downward to fill empty spaces, leaving empty cells at the top.

5. Analyze Complexity

State time complexity O(rows*cols) for BFS/DFS and gravity, and space complexity O(rows*cols) for visited set or recursion stack.

Key Points to Mention

Use BFS/DFS for connected component identification
In-place marking to save space
Two-pointer technique for efficient gravity application
Handling edge cases like empty grid or no groups meeting threshold
Time and space complexity analysis
Potential for cascading eliminations if required

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