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# 0529. Minesweeper

<https://leetcode.com/problems/minesweeper>

## Description

Let's play the minesweeper game ([Wikipedia](https://en.wikipedia.org/wiki/Minesweeper_\(video_game\)), [online game](http://minesweeperonline.com))!

You are given an `m x n` char matrix `board` representing the game board where:

* `'M'` represents an unrevealed mine,
* `'E'` represents an unrevealed empty square,
* `'B'` represents a revealed blank square that has no adjacent mines (i.e., above, below, left, right, and all 4 diagonals),
* digit (`'1'` to `'8'`) represents how many mines are adjacent to this revealed square, and
* `'X'` represents a revealed mine.

You are also given an integer array `click` where `click = [clickr, clickc]` represents the next click position among all the unrevealed squares (`'M'` or `'E'`).

Return *the board after revealing this position according to the following rules*:

1. If a mine `'M'` is revealed, then the game is over. You should change it to `'X'`.
2. If an empty square `'E'` with no adjacent mines is revealed, then change it to a revealed blank `'B'` and all of its adjacent unrevealed squares should be revealed recursively.
3. If an empty square `'E'` with at least one adjacent mine is revealed, then change it to a digit (`'1'` to `'8'`) representing the number of adjacent mines.
4. Return the board when no more squares will be revealed.

**Example 1:**

![](https://assets.leetcode.com/uploads/2018/10/12/minesweeper_example_1.png)

```
**Input:** board = [["E","E","E","E","E"],["E","E","M","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"]]
```

**Example 2:**

![](https://assets.leetcode.com/uploads/2018/10/12/minesweeper_example_2.png)

```
**Input:** 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"]]
```

**Constraints:**

* `m == board.length`
* `n == board[i].length`
* `1 <= m, n <= 50`
* `board[i][j]` is either `'M'`, `'E'`, `'B'`, or a digit from `'1'` to `'8'`.
* `click.length == 2`
* `0 <= clickr < m`
* `0 <= clickc < n`
* `board[clickr][clickc]` is either `'M'` or `'E'`.

## ac

```java
class Solution {
    int[][] dirs = {{0,1},{0, -1},{1, 1},{1, -1},{1, 0},{-1, 0},{-1, -1},{-1, 1}};

    public char[][] updateBoard(char[][] board, int[] click) {
        // hit mine
        if (board[click[0]][click[1]] == 'M') {
            board[click[0]][click[1]] = 'X';
            return board;
        }

        int row = board.length, col = board[0].length;
        boolean[][] visited = new boolean[row][col];

        Queue<int[]> q = new LinkedList<>();
        q.offer(click);
        while (!q.isEmpty()) {
            int[] curr = q.poll();
            int cnt = 0;
            List<int[]> next = new ArrayList<>();
            for (int[] d : dirs) {
                int r = curr[0] + d[0];
                int c = curr[1] + d[1];
                if (r < 0 || r >= row || c < 0 || c >= col) continue;
                if (board[r][c] == 'M') cnt++; // meet one mine 
                if (!visited[r][c] && board[r][c] == 'E') next.add(new int[]{r, c});
                visited[r][c] = true;
            }
            if (cnt == 0) { // empty safe cell
                board[curr[0]][curr[1]] = 'B';
                for (int[] t : next) q.offer(t);
            } else {
                board[curr[0]][curr[1]] = (char)('0'+cnt);
                for (int[] t : next) visited[t[0]][t[1]] = false; // release back
            }
        }

        return board;
    }
}

/*
BFS, 1) count neighbor mines, add unvisited cells to tmp list; 2) if cnt == 0, update blank cell, add tmp list to next visit; if cnt > 0, update mines number, set tmp list cells unvisited
*/
```