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"]]