130. Surrounded Regions

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.

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

class Solution {
    public void solve(char[][] board) {
        if(board.length <= 2 || board[0].length <= 2 ) 
            return;
        int row = board.length;
        int column = board[0].length;
        for(int i = 0; i < row; i++){
            if(board[i][0] == 'O'){
                boundarySearch(board, i , 0);
            }
            if(board[i][column - 1] == 'O'){
                boundarySearch(board, i, column-1);
            }
        }
        for(int j = 0; j < column; j++){
            if(board[0][j] == 'O'){
                boundarySearch(board, 0, j);
            }
            if(board[row-1][j] == 'O'){
                boundarySearch(board, row - 1, j);
            }
        }
        
        for(int i = 0; i < row; i++){
            for(int j = 0; j < column; j++){
                if(board[i][j] == 'O'){
                    board[i][j] = 'X';
                }else if(board[i][j] == '*'){
                    board[i][j] = 'O';
                }
            }
        }
    }
    
    public void boundarySearch(char[][] board, int i, int j){
        if(i < 0 || j < 0 || i >= board.length || j >= board[0].length) 
            return;
        if(board[i][j] == 'X' || board[i][j] == '*') 
            return;
        board[i][j] = '*';
        if(i > 1 && board[i - 1][j] == 'O') 
            boundarySearch(board, i - 1, j);
        if(j > 1 && board[i][j - 1] == 'O') 
            boundarySearch(board, i, j - 1);
        if(i < board.length-2 && board[i + 1][j] == 'O') 
            boundarySearch(board, i + 1, j);
        if(j < board[0].length-2 && board[i][j + 1] == 'O') 
            boundarySearch(board, i, j + 1);
    }
}

class Solution {
    public void solve(char[][] board) {
        
        if(board.length<2 || board[0].length<2){
            return;
        }
        
        for(int i =0 ; i < board.length; i ++){
            dfs(board, '*', i, 0);
            dfs(board, '*', i, board[0].length-1);
        }
        
        for(int i =0 ; i < board[0].length; i ++){
           dfs(board, '*', 0, i);
           dfs(board, '*', board.length-1, i);
        }
        
         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';
                }
            }
        }
        
        for(int i =0 ; i < board.length; i ++){ 
            for(int j = 0; j < board[0].length; j++){
                if(board[i][j] == '*'){
                    board[i][j] = 'O';
                }
            }
        }
    }
    
    public void dfs(char[][] board, char c, int i, int j){
        if( i>= board.length || i<0 || j >= board[0].length || j<0 ){
            return;
        }
        if(board[i][j] != 'O'){
            return;
        }
        
        board[i][j] = c;
        
        dfs(board, c, i+1,j);
        dfs(board, c, i,j+1);
        dfs(board, c, i-1,j);
        dfs(board, c, i,j-1);
    }
}