leetcode--Surrounded Regions

Given a 2D board containing 'X' and 'O', capture all regions surrounded by 'X'.

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

For 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

This is a simple problem, we just need to apply bfs on 'O' elements on the edges of board. 

public class Solution {
    public void solve(char[][] board) {
        int row = board.length;
		if(row > 0){
			int column = board[0].length;
			Set<Integer> alreadyChecked = new HashSet<Integer>();
			for(int i = 0; i < column; ++i){
				if(board[0][i] == 'O' && !alreadyChecked.contains(i))
					bfs(board,0, i, alreadyChecked);
				if(board[row - 1][i] == 'O' && !alreadyChecked.contains((row - 1) * column + i))
					bfs(board,row - 1, i, alreadyChecked);
			}
			for(int i = 1; i < row - 1; ++i){
				if(board[i][0] == 'O' && !alreadyChecked.contains(i * column))
					bfs(board,i, 0, alreadyChecked);
				if(board[i][column - 1] == 'O' && !alreadyChecked.contains(i * column + column - 1))
					bfs(board,i, column - 1, alreadyChecked);
			}
			
			for(int i = 0; i < row; ++i){
				for(int j = 0; j < column; ++j){
					if(board[i][j] == 'O' && !alreadyChecked.contains(i*column + j))
						board[i][j] = 'X';
				}
			}
		}
    }
	
	private void bfs(char[][] board, int i, int j, Set<Integer> alreadyChecked) {
		int row = board.length;
		if(row > 0){
			int column = board[0].length;
			if(!alreadyChecked.contains(i * column + j)) {
				Queue<Integer> current = new LinkedList<Integer>();
				alreadyChecked.add(i * column + j);
				current.add(i * column + j);
				while(current.peek() != null){
					int currentCoordinate = current.poll();
					int x = currentCoordinate / column;
					int y = currentCoordinate % column;
					
					if(x - 1 >= 0 && board[x - 1][y] == 'O' && !alreadyChecked.contains(currentCoordinate - column)){
						current.add(currentCoordinate - column);
						alreadyChecked.add(currentCoordinate - column);
					}
					
					if(x + 1 < row && board[x + 1][y] == 'O' && !alreadyChecked.contains(currentCoordinate + column)){
						current.add(currentCoordinate + column);
						alreadyChecked.add(currentCoordinate + column);
					}
					
					if(y - 1 >= 0 && board[x][y - 1] == 'O' && !alreadyChecked.contains(currentCoordinate - 1)) {
						current.add(currentCoordinate - 1);
						alreadyChecked.add(currentCoordinate - 1);
					}
					
					if(y + 1 < column && board[x][y + 1] == 'O' && !alreadyChecked.contains(currentCoordinate + 1)) {
						current.add(currentCoordinate + 1);
						alreadyChecked.add(currentCoordinate + 1);
					}
				}
			}
		}    
    }
}