Surrounded Regions

class Solution {
    public void solve(char[][] board) {
        if (board.length == 0) return;
        int r = board.length, c = board[0].length; 
        char toFillWith = '*';
        
        for (int i = 0; i < r; i++) {
            for (int j = 0; j < c; j++) {
                if (i == 0 || i == r - 1 || j == 0 || j == c - 1) {
                    dfs(board, i, j, 'O', toFillWith);
                }
            }
        }
        for (int i = 1; i < r - 1; i++) {
            for (int j = 1; j < c - 1; j++) {
                if (board[i][j] == 'O') {
                    dfs(board, i, j, 'O', 'X');
                }
            }
        }
        for (int i = 0; i < r; i++) {
            for (int j = 0; j < c; j++) {
                if (board[i][j] == toFillWith) {
                    dfs(board, i, j, toFillWith, 'O');
                }
            }
        }
        
        return;
    }
    
    public void dfs(char[][] board, int i, int j, char target, char fill) {
        if (i >= 0 && i < board.length && j >= 0 && j < board[i].length && board[i][j] == target) {
            board[i][j] = fill;
            dfs(board, i + 1, j, target, fill);
            dfs(board, i, j + 1, target, fill);
            dfs(board, i - 1, j, target, fill);
            dfs(board, i, j - 1, target, fill);
        }
        return;
    }
}