Onsite Coding: Minesweeper (LC 529)

An onsite coding round used the canonical LC 529 Minesweeper update task after a simpler undisclosed warm-up. Given a board and a click, update the board according to the mine and reveal rules. Requirements Input is a 2-D Minesweeper board plus the row and column of a click. An unrevealed mine changes to X when clicked. An unrevealed empty cell with one or more adjacent mines changes to the corresponding digit. An unrevealed empty cell with no adjacent mine changes to B, and the same reveal rule expands through its unrevealed neighbors. Return the updated board after the click has been fully processed. Notes The round used the canonical LC 529 task and placed a simpler warm-up before the main board-reveal step. The warm-up contract was not disclosed, so clarify how it connects to the main task before coding. Use BFS or DFS from the clicked cell. For each unrevealed empty cell, count mines across all eight neighboring positions; write the corresponding digit when the count is positive, otherwise write B and expand into neighboring unrevealed empty cells. Mark a cell when scheduling it for expansion, or mutate it before visiting its neighbors, so multiple blank parents cannot enqueue the same cell repeatedly. A direct mine click is terminal and changes only that cell to X. Worst-case time is O(mn) because each cell is processed at most once. The BFS queue or recursive DFS stack can use O(mn) auxiliary space. Preparation Implement both BFS and DFS versions from scratch, keeping the eight directions in one reusable table and marking cells before enqueueing or recursing. Run a focused test set covering a direct mine click, a numbered cell next to a diagonal mine, a blank cascade at an edge or corner, and multiple blank parents reaching the same unrevealed cell.