class Solution:
    def solve(self, board: List[List[str]]) -> None:
        rows, cols = len(board), len(board[0])
        flag = set()

        def dfs(r, c):
            if not(r in range(rows) and c in range(cols)) or board[r][c] != 'O' or (r, c) in flag:
                return
            flag.add((r, c))
            return (dfs(r + 1, c), dfs(r - 1, c), dfs(r, c + 1), dfs(r, c - 1))

        # traverse through the board
        for r in range(rows):
            for c in range(cols):
                if( (r == 0 or c == 0 or r == rows - 1 or c == cols - 1) and board[r][c] == 'O'):
                    dfs(r, c)

        # set all of the 'X's to 'O's
        for r in range(rows):
            for c in range(cols):
                if board[r][c] == 'O' and (r, c) not in flag:
                    board[r][c] = 'X'

    '''
    def solve(self, board: List[List[str]]) -> None:
        ROWS, COLS = len(board), len(board[0])

        def capture(r, c):
            if r < 0 or c < 0 or r == ROWS or c == COLS or board[r][c] != "O":
                return
            board[r][c] = "T"
            capture(r + 1, c)
            capture(r - 1, c)
            capture(r, c + 1)
            capture(r, c - 1)

        # 1. (DFS) Capture unsurrounded regions (O -> T)
        for r in range(ROWS):
            for c in range(COLS):
                if board[r][c] == "O" and (r in [0, ROWS - 1] or c in [0, COLS - 1]):
                    capture(r, c)

        # 2. Capture surrounded regions (O -> X)
        for r in range(ROWS):
            for c in range(COLS):
                if board[r][c] == "O":
                    board[r][c] = "X"

        # 3. Uncapture unsurrounded regions (T -> O)
        for r in range(ROWS):
            for c in range(COLS):
                if board[r][c] == "T":
                    board[r][c] = "O"
    '''