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 XX O O XX X O XX O X X

After running your function, the board should be:

X X X XX X X XX X X XX O X X

public class Solution {    public void solve(char[][] board) {// Start typing your Java solution below// DO NOT write main() functionif(board.length < 1)return;//change 'O' to 'A'//left -> rightfor(int j = 0; j < board[0].length; j++)flip(board, 0, j);//up -> downfor(int i = 0; i < board.length; i++)flip(board, i, board[0].length - 1);//right -> leftfor(int j = board[0].length - 1; j >= 0; j--)flip(board, board.length - 1, j);//down -> upfor(int i = board.length - 1; i >= 0; i--)flip(board, i, 0);//change 'O' to 'X'for(int i = 0; i < board.length; i++)for(int j = 0; j < board[0].length; j++)if(board[i][j] == 'O')board[i][j] = 'X';//change 'A' to 'O'for(int i = 0; i < board.length; i++)for(int j = 0; j < board[0].length; j++)if(board[i][j] == 'A')board[i][j] = 'O';}public void flip(char[][] board, int i, int j){if(i < 0 || i >= board.length || j < 0 || j >= board[0].length)return;if(board[i][j] == 'O'){board[i][j] = 'A';flip(board, i - 1, j);flip(board, i + 1, j);flip(board, i, j - 1);flip(board, i, j + 1);}}}