130. Surrounded Regions

public void solve(char[][] board) {
    if (board == null || board.length == 0) return;
    int rows = board.length, cols = board[0].length;
    boolean[][] visited = new boolean[rows][cols];

    for (int j = 0; j < cols; j++) {
        if (board[0][j] == 'O') {
            dfs(board, 0, j, visited, false);
        }

        if (board[rows - 1][j] == 'O') {
            dfs(board, rows - 1, j, visited, false);
        }
    }

    for (int i = 0; i < rows; i++) {
        if (board[i][0] == 'O') {
            dfs(board, i, 0, visited, false);
        }

        if (board[i][cols - 1] == 'O') {
            dfs(board, i, cols - 1, visited, false);
        }
    }

    for (int i = 1; i < rows - 1; i++) {
        for (int j = 1; j < cols - 1; j++) {
            if (board[i][j] == 'O' && !visited[i][j]) {
                dfs(board, i, j, visited, true);
            }
        }
    }
}

private void dfs(char[][] board, int row, int col, 
    boolean[][] visited, boolean flip) {
    if (row < 0 || col < 0 || row > board.length - 1 ||
        col > board[0].length - 1) return;
    if (visited[row][col]) return;
    if (board[row][col] == 'X') return;
    if (flip) {
        board[row][col] = 'X';
    }

    visited[row][col] = true;
    dfs(board, row + 1, col, visited, flip);
    dfs(board, row - 1, col, visited, flip);
    dfs(board, row, col + 1, visited, flip);
    dfs(board, row, col - 1, visited, flip);
}