130. Surrounded Regions

1/**
2 Do not return anything, modify board in-place instead.
3 */
4function solve(board: string[][]): void {
5    if (board.length === 0) return;
6    const rows = board.length;
7    const cols = board[0].length;
8    const dirs = [
9        [1, 0], [0, 1], [0, -1], [-1, 0]
10    ]
11
12    function dfs(row: number, col: number): void {
13        if (row < 0 || row >= rows || col < 0 || col >= cols || board[row][col] !== 'O') return;
14        board[row][col] = '#'
15        for (const [dx, dy] of dirs) {
16            const new_r = dx + row;
17            const new_c = dy + col;
18            dfs(new_r, new_c)
19        }
20    }
21
22    for (let i = 0; i < cols; i++) {
23        dfs(0, i);
24        dfs(rows - 1, i)
25    }
26
27    for (let i = 0; i < rows; i++) {
28        dfs(i, 0);
29        dfs(i, cols - 1)
30    }
31
32    for (let i = 0; i < rows; i++) {
33        for (let j = 0; j < cols; j++) {
34            if (board[i][j] === 'O') {
35                board[i][j] = 'X'
36            } else if (board[i][j] === '#') {
37                board[i][j] = 'O'
38            }
39        }
40    }
41};