SurroundedRegions.py

# Question Link - https://leetcode.com/problems/surrounded-regions/description/?envType=study-plan-v2&envId=top-interview-150

class Solution:
    def solve(self, board: List[List[str]]) -> None:
        """
        Do not return anything, modify board in-place instead.
        """
        # Find the rows and cols of the Matrix
        ROWS  , COLS = len(board) , len(board[0])

        # DFS , function for the capture 'O' -> 'T'
        def capture(r , c):
            # Base case to return 
            if (r < 0 or c < 0 or r == ROWS or c == COLS or 
                board[r][c] != 'O'):
                return 
            board[r][c] = 'T'
            # Search for 4 directions
            capture(r + 1, c)
            capture(r - 1, c)
            capture(r ,c + 1)
            capture(r ,c - 1)

        # 1st pahse , convert the boundary 0 to 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)

        # 2nd phase , convert the 0 to X as its not in boundary 
        for r in range(ROWS):
            for c in range(COLS):
                if board[r][c] == 'O':
                    board[r][c] = 'X'

        # 3rd phase , convert the T to O ,as original position 
        for r in range(ROWS):
            for c in range(COLS):
                if board[r][c] == 'T':
                    board[r][c] = 'O'