문제는 다음과 같습니다.

https://www.acmicpc.net/problem/14502

　
import java.util.Scanner;

public class Test14502 {
	static int N, M;
	static int totalCNT;
	static int[] dx = {0, -1, 0, 1};
	static int[] dy = {-1, 0, 1, 0};
	static int[][] adMatrix, visit;
	
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		N = sc.nextInt();
		M = sc.nextInt();
		adMatrix = new int[N][M];
		visit = new int[N][M];
		for(int i=0; i<N; i++) {
			for(int j=0; j<M; j++) {
				adMatrix[i][j] = sc.nextInt();
			}
		}
		//알고리즘 풀이
		for(int i=0; i<N*M; i++) {
			int row = i/M;
			int col = i%M;
			if(adMatrix[row][col]==0) {
				adMatrix[row][col]=1;
				DFS(i, 1);
				adMatrix[row][col]=0;
			}
		}
		System.out.println(totalCNT);
	}
	//벽 3개 세우기
	static void DFS(int v, int cnt) {
		int row = v/M;
		int col = v%M;
		if(cnt==3) {
			for(int i=0; i<N; i++) {
				for(int j=0; j<M; j++) {
					visit[i][j]=adMatrix[i][j];
				}
			}
			for(int i=0; i<N; i++) {
				for(int j=0; j<M; j++) {
					if(visit[i][j]==2) {
						spreadVirus(i, j);
					}
				}
			}
			safetyArea();
		}else {
			for(int i=v+1; i<N*M; i++) {
				int row2 = i/M;
				int col2 = i%M;
				if(adMatrix[row2][col2]==0) {
					adMatrix[row2][col2]=1;
					DFS(i, cnt+1);
				}
			}
		}
		adMatrix[row][col]=0;
	}
	//바이러스 확산 메소드
	static void spreadVirus(int row, int col) {
		for(int i=0; i<4; i++) {
			int nextRow = dx[i]+row;
			int nextCol = dy[i]+col;
			if(nextRow>=0 && nextRow<N && nextCol>=0 && nextCol<M) {
				if(visit[nextRow][nextCol]==0) {
					visit[nextRow][nextCol]=2;
					spreadVirus(nextRow, nextCol);
				}
			}
		}
	}
	//안정 영역 크기 구하기
	static void safetyArea() {
		int cnt=0;
		for(int i=0; i<N; i++) {
			for(int j=0; j<M; j++) {
				if(visit[i][j]==0) {
					cnt++;
				}
			}
		}
		if(totalCNT<cnt) {
			totalCNT = cnt;
		}
	}
}

　

결과입니다.