Color Match-Three Grid

Given a numeric matrix where each value represents a color, repeatedly explode every cell whose 4-neighborhood contains at least two same-colored cells, then let surviving cells fall under gravity. Return the resulting matrix. Requirements Input: an n x m integer matrix; each value is a color. A cell at (r, c) is marked for elimination if at least two of its 4-neighbors (up / down / left / right, within bounds) share its color. Mark all qualifying cells in one pass, then remove them simultaneously (do not cascade mid-pass; this is a single-step elimination). After removal, surviving cells fall straight down within their column so empty cells end up at the top. Return the resulting matrix. Notes Two-phase pattern: (1) iterate over all cells, compute a same-color neighbour count, mark those with count ≥ 2 in a boolean grid; (2) for each column, collect surviving values bottom-up and re-emit with leading zeros / sentinel above. The prompt does not specify whether the process repeats until no further eliminations are possible. The default interpretation is one round; if the interviewer extends it to cascade, wrap the two phases in a while changed loop. Edge cells have fewer neighbours; the threshold is ≥ 2 same-colored existing neighbours, not 2 out of 4. Gravity is per-column and stable (preserves the relative order of surviving cells). Preparation Implement the two-phase pattern cleanly on a 3x3 sample with a corner cluster, an isolated cell, and an edge case. If the interviewer extends to cascading rounds, the wrapping loop typically terminates in O(n·m) rounds for adversarial inputs; add a guard against infinite loops just in case.