Surrounded Regions

surrounded_regions.py

class Solution:
    def solve(self, board: List[List[str]]) -> None:
        if not board or len(board) == 0:
            return
        
        topRow, bottomRow = 0, len(board) - 1
        leftBorder, rightBorder = 0, len(board[0]) - 1
       
        def inBounds(i, j):
            return i >= topRow and i <= bottomRow and j >= leftBorder and j <= rightBorder
        def boundaryDfs(i, j):
            if not inBounds(i, j):
                return
            if board[i][j] == 'O':
                board[i][j] = '*'
            arr =[(i-1,j), (i+1, j), (i, j-1), (i, j+1)]

            for (x, y) in arr:
                if inBounds(x, y) and board[x][y] == 'O':
                    boundaryDfs(x, y)

        for n in range(len(board[0])):
            if board[0][n] == 'O':
                boundaryDfs(0, n)
            if board[bottomRow][n] == 'O':
                boundaryDfs(bottomRow, n)
        
        for m in range(len(board)):
            if board[m][0] == 'O':
                boundaryDfs(m, 0)
            if board[m][rightBorder] == 'O':
                boundaryDfs(m, rightBorder)

        for m in range(len(board)):
            for n in range(len(board[0])):
                if board[m][n] == 'O':
                    board[m][n] = 'X'
                elif board[m][n] == '*':
                    board[m][n] = 'O'