Surrounded Regions
Created: March 25, 2020 by [lek-tin]
Last updated: March 25, 2020
Given a 2D board containing 'X' and 'O' (the letter O), capture all regions surrounded by 'X'.
A region is captured by flipping all 'O's into 'X's in that surrounded region.
Example
X X X X
X O O X
X X O X
X O X X
After running your function, the board should be:
X X X X
X X X X
X X X X
X O X X
Explanation
Surrounded regions shouldn’t be on the border, which means that any 'O' on the border of the board are not flipped to 'X'. Any 'O' that is not on the border and it is not connected to an 'O' on the border will be flipped to 'X'. Two cells are connected if they are adjacent cells connected horizontally or vertically.
Solution
class Solution:
def solve(self, board: List[List[str]]) -> None:
"""
Do not return anything, modify board in-place instead.
"""
if not any(board):
return
n, m = len(board), len(board[0])
q = [ij for k in range(max(n,m)) for ij in ((0, k), (n-1, k), (k, 0), (k, m-1))]
while q:
i, j = q.pop()
if 0 <= i < n and 0 <= j < m and board[i][j] == 'O':
board[i][j] = 'W'
q += (i, j-1), (i, j+1), (i-1, j), (i+1, j)
board[:] = [['XO'[c == 'W'] for c in row] for row in board]