Compute reachable cells for a cleaning robot

You are given a 2D grid representing a floor plan: - `0` = free cell (the robot may stand on it) - `1` = blocked cell (obstacle) The robot can move from a cell to any of its **8 neighbors** (up, down, left, right, and 4 diagonals) as long as the destination cell is allowed under the rules of the sub-question. Return answers as a set/list of reachable coordinates `(r, c)` (order does not matter). ### Sub-question 1 (basic reachability) **Input:** grid `m x n` and a start cell `(sr, sc)` that is free. **Task:** return **all free cells** reachable from `(sr, sc)` using 8-direction moves. ### Sub-question 2 (one obstacle can be cleared) Same as Sub-question 1, except the robot may **convert at most one blocked cell (`1`) into free (`0`)** during its traversal (think: it can clear/remove one obstacle exactly once). **Task:** return all cells that are reachable under this rule. ### Sub-question 3 (two robots) **Input:** grid `m x n` and two start cells `(s1r, s1c)` and `(s2r, s2c)` that are free. **Task:** return the set of cells that are reachable by **at least one** of the two robots (union of their reachable areas) using 8-direction moves. ### Constraints (assume) - `1 <= m, n <= 2000` (choose an approach that is safe for large grids) - Grid is not necessarily fully connected. - Start cells are within bounds. Clarify in the interview how to represent the returned set for large outputs (e.g., boolean mask vs coordinate list).