Day_17_SurroundedRegions.java

class Solution {
    
    public void solve(char[][] board) {
        
        if(board.length<3 || board[0].length<3) return;
        
        int m=board.length;
        int n=board[0].length;
        
        for(int i=0;i<m;i++){
            
            if(board[i][0]=='O'){
                dfs(board,i,0);
                
            }
            if(board[i][n-1]=='O'){
                dfs(board,i,n-1);
            }
        }
        
        for(int i=0;i<n;i++){
            
            if(board[0][i]=='O'){
                dfs(board,0,i);
            }
            if(board[m-1][i]=='O'){
                dfs(board,m-1,i);
            }
        }
        
        for(int i=0;i<m;i++){
            for(int j=0;j<n;j++){
                
               if(board[i][j]=='O') board[i][j]='X';
                if(board[i][j]=='Y') board[i][j]='O';
            }
        }
        
        
    }
    
    public void dfs(char[][] board,int i,int j){
        
        if(i>=0 && j>=0 && i<board.length && j<board[i].length && board[i][j]=='O'){
            board[i][j]='Y';
            dfs(board,i+1,j);
            dfs(board,i-1,j);
            dfs(board,i,j+1);
            dfs(board,i,j-1);
        }
    }
}