# Surrounded Regions Python

Suppose we have a 2D board containing X and O. Capture all regions surrounded by X. A region is captured by changing all Os into Xs in that surrounded region.

 X X X X X O O X X X O X X O X X

After running the output will be

 X X X X X X X X X X X X X O X X

To solve this, we will follow these steps −

• If board is not present, then return blank board

• for i in range 0 to number of rows - 1 −

• if board[i, 0] = ‘O’, then make_one(board, i, 0)

• if board[i, length of row - 1] = ‘O’, then make_one(board, i, length of row - 1)

• for i in range 0 to number of cols - 1 −

• if board[0, i] = ‘O’, then make_one(board, 0, i)

• if board[count of row - 1, i] = ‘O’, then make_one(board, count of row - 1, i)

• for i in range 0 to number of rows

• for j in range 0 to number of cols

• if board[i, j] = ‘O’, then board[i, j] = ‘X’, otherwise for 1, board[i, j] = ‘O’

• The make_one will be like −

• if i < 0 or j < 0 or i >= row count or j >= col count or board[i, j] = ‘X’ or board[i, j] = ‘1’, then return

• board[i, j] := 1

• call make_one(voard, i + 1, j), make_one(voard, i - 1, j), make_one(voard, i, j + 1), make_one(voard, i, j - 1)

## Example

Let us see the following implementation to get better understanding −

class Solution(object):
def solve(self, board):
"""
:type board: List[List[str]]
:rtype: None Do not return anything, modify board in-place instead.
"""
if not board:
return board
for i in range(len(board)):
if board[i][0]=='O':
self.make_one(board,i,0)
if board[i][len(board[0])-1] == 'O':
self.make_one(board,i,len(board[0])-1)
for i in range(len(board[0])):
if board[0][i]=='O':
self.make_one(board,0,i)
if board[len(board)-1][i] == 'O':
self.make_one(board,len(board)-1,i)
for i in range(len(board)):
for j in range(len(board[i])):
if board[i][j]=='O':
board[i][j]='X'
elif board[i][j]=='1':
board[i][j]='O'
def make_one(self, board,i,j):
if i<0 or j<0 or i>=len(board) or j>=len(board[0]) or board[i][j]=='X' or board[i][j]=='1':
return
board[i][j]='1'
self.make_one(board,i+1,j)
self.make_one(board,i-1,j)
self.make_one(board,i,j+1)
self.make_one(board,i,j-1)

## Input

[["X","X","X","X"],["X","O","O","X"],["X","X","O","X"],["X","O","X","X"]]

## Output

[["X","X","X","X"],["X","X","X","X"],["X","X","X","X"],["X","O","X","X"]]

Updated on: 03-Feb-2020

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