Reveal a Minesweeper Region with Adjacent-Mine Counts

Problem Statement

In the game of Minesweeper, a board is represented as an m x n 2D grid of characters where each square has one of the following states:

• 'M': An unrevealed mine.
• 'E': An unrevealed empty square.
• 'B': A revealed blank square that has no adjacent mines (all 8 neighbors are mine-free).
• '1' to '8': A revealed square indicating the exact number of adjacent mines.
• 'X': A revealed mine (representing a game-over trigger).

You are given the current board and a target cell click = [r, c] representing the coordinate clicked by the player. Implement a function to update and return the board according to the following rules:

1. If a mine ('M') is clicked, change its state to 'X' and immediately return the updated board (game over).
2. If an empty square ('E') with at least one adjacent mine (in any of the 8 directions) is clicked, change its state to the digit character representing the count of adjacent mines ('1' to '8'). Do not reveal any neighbors.
3. If an empty square ('E') with zero adjacent mines is clicked, change its state to 'B'. Then, recursively reveal all of its 8-directionally adjacent unrevealed squares ('E').
4. If the clicked square is already revealed ('B', '1'–'8', or 'X'), leave the board unchanged.

Function Signature

def reveal_board(board: list[list[str]], click: list[int]) -> list[list[str]]:
    pass

Constraints

• m == len(board)
• n == len(board[0])
• 1 <= m, n <= 1000
• 1 <= m * n <= 100000
• board[i][j] is one of 'M', 'E', 'B', or '1'–'8'.
• click is a 2-element list [r, c] where 0 <= r < m and 0 <= c < n.

Examples

Example 1

board = [
    ["E", "E", "E", "E", "E"],
    ["E", "M", "E", "E", "E"],
    ["E", "E", "E", "E", "E"],
    ["E", "E", "E", "E", "E"]
]
click = [3, 0]

# Output:
# [
#     ["B", "1", "E", "1", "B"],
#     ["B", "1", "M", "1", "B"],
#     ["B", "1", "1", "1", "B"],
#     ["B", "B", "B", "B", "B"]
# ]

Explanation: Cell [3, 0] has 0 adjacent mines, so it becomes 'B'. The reveal propagates to adjacent cells. Cells adjacent to 'M' (such as [0, 1], [1, 0], etc.) stop the propagation and display their adjacent mine counts.

Example 2

board = [
    ["B", "1", "E", "1", "B"],
    ["B", "1", "M", "1", "B"],
    ["B", "1", "1", "1", "B"],
    ["B", "B", "B", "B", "B"]
]
click = [1, 2]

# Output:
# [
#     ["B", "1", "E", "1", "B"],
#     ["B", "1", "X", "1", "B"],
#     ["B", "1", "1", "1", "B"],
#     ["B", "B", "B", "B", "B"]
# ]

Explanation: The user clicked a mine at [1, 2]. It turns to 'X' and no other cells are modified.