[ OK ]afae04f7-af09-4754-aaf5-58509e3ad372 — full content available
[ INFO ]category: Coding difficulty: unknown freq: first seen: 2026-03-13
[UNKNOWN][CODING]
$catproblem.md
While the specific "Grid Drop and Remove Duplicates" title isn't a standard LeetCode name, it is a real-world interview problem recently reported at companies like xAI and Snowflake. 3
Based on technical interview reports from March 2026, the problem is a variation of Connect-4 or Candy Crush logic that tests your ability to handle grid manipulation and gravity. 3
Problem Statement (General Version)
You are given an 𝑚×𝑛 grid representing a game board. Each cell contains either a value (representing a piece/color) or is empty. You must implement a utility that performs two primary actions: 3
Drop Pieces: Simulate gravity. If a cell is empty and there are pieces above it in the same column, those pieces "drop" down to fill the empty spaces.
Remove Duplicates (Matches): Identify and remove "duplicate" pieces that meet a specific condition (usually 3 or more of the same value in a row or column).
Iterative Process: After pieces are removed, new empty spaces are created. The pieces above must drop again, potentially creating new matches. The process continues until no more matches are found.
Key Requirements & Constraints
Grid Dimensions: Typically 𝑚×𝑛 where 𝑚,𝑛≤100.
In-Place vs. New Grid: You may be asked to modify the grid in-place or return a new state.
Stable Gravity: When pieces drop, their relative vertical order in a column must be preserved.
Efficiency: The interviewer often looks for a solution that avoids unnecessary re-scans of the entire grid. DEV Community +3
Recommended Implementation Steps
Gravity Function: Use a "two-pointer" approach for each column. One pointer tracks the next available "empty" slot at the bottom, and the other scans upward for the next available piece to move down.
Match Detection: Use a traversal (DFS or simple iteration) to mark all pieces that are part of a matching sequence (e.g., 3+ identical adjacent values).
Find matches →right arrow→ Mark for removal.
If no matches found →right arrow→ Return.
Remove marked pieces →right arrow→ Apply gravity.
Repeat.
Find matches →right arrow→ Mark for removal.
If no matches found →right arrow→ Return.
Remove marked pieces →right arrow→ Apply gravity.
Repeat.
Expected Follow-ups at xAI
Boundary Handling: How do you handle matches that wrap around or reach the edge of the board?
Optimization: Can you use a Trie or Hash Map if the "duplicates" are based on complex patterns rather than just identical values?
Concurrency: If the grid was massive, how would you parallelize the gravity or matching logic across multiple rows/columns? DEV Community +1
Would you like a Python code template for the gravity and matching logic?