Leetcode 130. Surrounded Regions

Problem

Given an m x n matrix board containing ‘X’ and ‘O’, capture all regions that are 4-directionally surrounded by ‘X’.

A region is captured by flipping all 'O’s into 'X’s in that surrounded region.

Algorithm

Recursion. Set all the “O” to “#” and change them back if some point can be visit from the edges.

Code

class Solution:
    def solve(self, board: List[List[str]]) -> None:
        """
        Do not return anything, modify board in-place instead.
        """
        if not board:
            return []
        
        row = len(board)
        col = len(board[0])
        
        def dfs(r, c, i, j):
            if i < 0 or i >= r or j < 0 or j >= c:
                return
            if board[i][j] == "O":
                board[i][j] = "#"
                dfs(r, c, i-1, j)
                dfs(r, c, i+1, j)
                dfs(r, c, i, j-1)
                dfs(r, c, i, j+1)
        
        for r in range(row):
            dfs(row, col, r, 0)
            dfs(row, col, r, col-1)
        
        for c in range(col):
            dfs(row, col, 0, c)
            dfs(row, col, row-1, c)
        
        for r in range(row):
            for c in range(col):
                if board[r][c] == "O":
                    board[r][c] = "X"
                if board[r][c] == "#":
                    board[r][c] = "O"