테스트케이스는 잘 되는데 제출하면 에러

#include <bits/stdc++.h>

using namespace std;

int N, M; // 세로 : N
int arr[8][8];
int visited[8][8];
int dx[] = { 0, 1, 0, -1 };
int dy[] = { -1, 0, 1, 0 };
int t = 1;
int curVirus = 2;
int cnt = 0;
int maxCnt = 0;

void BFS(int _y, int _x)
{
	queue<pair<int, int>> q;

	q.push({ _y, _x });
	int firstX = _x;
	int firstY = _y;

	while (q.size())
	{
		tie(_y, _x) = q.front();
		visited[_y][_x] = t;
		arr[_y][_x] = curVirus;
		q.pop();

		for (int i = 0; i < 4; ++i)
		{

			int nx = _x + dx[i];
			int ny = _y + dy[i];

			if (nx < 0 || ny < 0 || nx >= M || ny >= N) continue;
			if (arr[ny][nx] == 0 && visited[ny][nx] != t)
				q.push({ ny, nx });
		}
	}
	arr[firstY][firstX] = curVirus + 1;
}

void ClearSpreadVirus()
{
	for (int y = 0; y < N; ++y)
	{
		for (int x = 0; x < M; ++x)
		{
			if (arr[y][x] == curVirus) arr[y][x] = 0;
		}
	}
}

void SpreadVirus()
{
	for (int y = 0; y < N; ++y)
	{
		for (int x = 0; x < M; ++x)
		{
			if (arr[y][x] != curVirus) continue;
			if (visited[y][x] == t) continue;

			BFS(y, x);
		}
	}
}

void CountZero()
{
	for (int y = 0; y < N; ++y)
	{
		for (int x = 0; x < M; ++x)
		{
			if (arr[y][x] == 0) ++cnt;
		}
	}
}

int main()
{
	ios::sync_with_stdio(false); cin.tie(nullptr); cout.tie(nullptr);
	
	cin >> N >> M;
	for (int y = 0; y < N; ++y)
	{
		for (int x = 0; x < M; ++x)
		{
			cin >> arr[y][x];
		}
	}

	// 1. 브루트포스로 벽을 세운다.
	for (int b1 = 0; b1 < N * M; ++b1)
	{
		if (arr[b1 / M][b1 % M] != 0) continue;

		arr[b1 / M][b1 % M] = 1;

		for (int b2 = b1 + 1; b2 < N * M - 1; ++b2)
		{
			if (arr[b2 / M][b2 % M] != 0) continue;

			arr[b2 / M][b2 % M] = 1;

			for (int b3 = b2 + 1; b3 < N * M - 2; ++b3)
			{
				if (arr[b3 / M][b3 % M] != 0) continue;

				arr[b3 / M][b3 % M] = 1;

				SpreadVirus();
				CountZero();
				ClearSpreadVirus();
				++curVirus;
				maxCnt = max(maxCnt, cnt);
				cnt = 0;
				++t;

				arr[b3 / M][b3 % M] = 0;
			}
			arr[b2 / M][b2 % M] = 0;
		}
		arr[b1 / M][b1 % M] = 0;
	}
	cout << maxCnt;

	return 0;
}

안녕하세요.

예제케이스는 답이 잘 나오는데 제출하면 에러가 뜹니다.

코드가 복잡해서 디버깅이 쉽지 않네요..

어디가 문제일까요?

	for (int b1 = 0; b1 < N * M; ++b1)
	{
		if (arr[b1 / M][b1 % M] != 0) continue;

		arr[b1 / M][b1 % M] = 1;

		for (int b2 = b1 + 1; b2 < N * M; ++b2)
		{
			if (arr[b2 / M][b2 % M] != 0) continue;

			arr[b2 / M][b2 % M] = 1;

			for (int b3 = b2 + 1; b3 < N * M; ++b3)