← Boston Consulting Group Interview Insights

Boston Consulting Group·AI Engineer·Technical Phone Screen·Intermediate

IntermediatePrefer not to say
Jun 2026

Summary

Coding round for an AI Engineer role at BCG. One algorithmic problem, grid-based, heavier on simulation logic than anything else.

Questions Asked (1)

Q1

You're given a 2D grid of colored bubbles. Any bubble that has at least 2 neighbors (up, down, left, right) of the same color gets popped simultaneously with all other such bubbles. After popping, those cells become 0 and the remaining bubbles above each column fall down. Implement this.

Algorithms & Data Structures
Author's notes

The simultaneous part is what tripped me up at first.

Create a free account to read the full note

AI HintsAI Generated

Suggested Approach

Clarify the rules and edge cases, then outline a two-phase simulation: first identify all bubbles to pop using BFS/DFS or a scan, then remove them and apply gravity column-wise. Discuss time/space complexity and potential optimizations like union-find or iterative elimination.

Pro tip: Mention that simultaneous popping requires marking cells before removal to avoid cascading effects, and that gravity can be implemented efficiently with a two-pointer technique per column.

1. Clarify requirements and edge cases

Ask about grid dimensions, color representation, whether diagonal neighbors count, and if multiple rounds of popping occur. Confirm that popping is simultaneous and gravity applies after each round.

2. Identify bubbles to pop

Scan the grid and for each unvisited bubble, perform BFS/DFS to find its connected component of same-colored neighbors. If size >= 3 (including itself), mark all cells in the component for popping.

3. Remove marked bubbles and apply gravity

Set marked cells to 0. Then for each column, compact non-zero values downward using a two-pointer approach (write pointer from bottom, read pointer from bottom).

4. Repeat if necessary

If the problem requires cascading pops, repeat steps 2-3 until no more bubbles can be popped. Otherwise, return the updated grid.

5. Analyze complexity and discuss optimizations

State time complexity O(R*C) per round and space O(R*C) for visited/marked arrays. Mention potential optimizations like union-find for component detection or in-place marking.

Key Points to Mention

  • Simultaneous popping: mark all cells to pop before actually removing them to avoid affecting neighbor checks.
  • Connected components: use BFS/DFS to find groups of same-colored bubbles with size >= 3.
  • Gravity implementation: two-pointer technique per column for O(R*C) compaction.
  • Edge cases: empty grid, no pops, all same color, single row/column.
  • Time and space complexity: O(R*C) per round, O(R*C) space for visited/marked arrays.
  • Potential optimizations: union-find for component detection, iterative elimination with early termination.

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