dsa/Graphs/SurroundedRegions.java

// https://leetcode.com/problems/surrounded-regions/

import java.util.*;
import java.io.*;

class Main {

    static class Node{
        int row;
        int col;

        Node(int row, int col){
            this.row = row;
            this.col = col;
        }
    }

    // DFS
    public static void dfs(Node node, char board[][], boolean doNotTouch[][]){

        int m = board.length;
        int n = board[0].length;

        int r = node.row;
        int c = node.col;

        doNotTouch[r][c] = true;

        int dr[] = {-1, 0, 1, 0};
        int dc[] = {0, 1, 0, -1};

        for(int k=0;k<4;k++){
            int nr = r + dr[k];
            int nc = c + dc[k];

            if(nr>=0 && nc>=0 && nr<m && nc<n &&
               !doNotTouch[nr][nc] && board[nr][nc]=='O'){
                dfs(new Node(nr, nc), board, doNotTouch);
            }
        }
    }


    // BFS
    public static void bfs(Node start, char board[][], boolean doNotTouch[][]){

        int m = board.length;
        int n = board[0].length;

        Queue<Node> q = new LinkedList<>();
        q.add(start);
        doNotTouch[start.row][start.col] = true;

        int dr[] = {-1, 0, 1, 0};
        int dc[] = {0, 1, 0, -1};

        while(!q.isEmpty()){
            Node cur = q.poll();

            for(int k=0;k<4;k++){
                int nr = cur.row + dr[k];
                int nc = cur.col + dc[k];

                if(nr>=0 && nc>=0 && nr<m && nc<n &&
                   !doNotTouch[nr][nc] && board[nr][nc]=='O'){
                    doNotTouch[nr][nc] = true;
                    q.add(new Node(nr, nc));
                }
            }
        }
    }


    public static void solve(char board[][]){

        int m = board.length;
        int n = board[0].length;

        boolean doNotTouch[][] = new boolean[m][n];

        // borders
        for(int i=0;i<m;i++){
            if(board[i][0]=='O'){
                dfs(new Node(i,0), board, doNotTouch);
                // bfs(new Node(i,0), board, doNotTouch);
            }
            if(board[i][n-1]=='O'){
                dfs(new Node(i,n-1), board, doNotTouch);
                // bfs(new Node(i,n-1), board, doNotTouch);
            }
        }

        for(int j=0;j<n;j++){
            if(board[0][j]=='O'){
                dfs(new Node(0,j), board, doNotTouch);
                // bfs(new Node(0,j), board, doNotTouch);
            }
            if(board[m-1][j]=='O'){
                dfs(new Node(m-1,j), board, doNotTouch);
                // bfs(new Node(m-1,j), board, doNotTouch);
            }
        }

        char finalAns[][] = new char[m][n];

        for(int i=0;i<m;i++){
            for(int j=0;j<n;j++){
                if(doNotTouch[i][j]) finalAns[i][j] = board[i][j];
                else finalAns[i][j] = 'X';
            }
        }

        // copy back
        for(int i=0;i<m;i++){
            for(int j=0;j<n;j++){
                board[i][j] = finalAns[i][j];
            }
        }
    }


    // DRIVER
    public static void main(String args[]){

        char board[][] = {
            {'X','X','X','X'},
            {'X','O','O','X'},
            {'X','X','O','X'},
            {'X','O','X','X'}
        };

        solve(board);

        for(int i=0;i<board.length;i++){
            for(int j=0;j<board[0].length;j++){
                System.out.printf("%c ", board[i][j]);
            }
            System.out.printf("\n");
        }
    }
}