class Solution:
    def solve(self, board: List[List[str]]) -> None:
        """
        Do not return anything, modify board in-place instead.
        """

        rows = len(board)
        cols = len(board[0])
        visited = set()

        def dfs(r, c):
            if r<0 or c<0 or c == cols or r == rows or board[r][c] == "X" or (r, c) in visited:
                return

            visited.add((r, c))

            dfs(r+1, c)
            dfs(r-1, c)
            dfs(r, c+1)
            dfs(r, c-1)

            return

        for r in range(rows):
            dfs(r, 0)
            dfs(r, cols-1)

        for c in range(cols):
            dfs(0, c)
            dfs(rows-1, c)

        for r in range(rows):
            for c in range(cols):
                if (r, c) not in visited:
                    board[r][c] = "X"