use one dimension array to store the value, and no queue, no recursive call; confused, need to figure out how it is designed.

ConnectedCellInaGrid5.cpp

#include <cmath>
#include <cstdio>
#include <vector>
#include <iostream>
#include <algorithm>
#include <cstring>
using namespace std;

const int MAXN = 111;

int Set[MAXN];

void init_set() {
	memset(Set, -1, sizeof(Set));
}
int find_set(int x) {
	while (Set[x] >= 0) {
		//Set[x] = find_set(Set[x]);//
		x = Set[x];
	}
	return x;
}
int union_set(int x, int y) {
	int fx = find_set(x);
	int fy = find_set(y);
	if (fx == fy)
		return 0;
	Set[fx] += Set[fy];
	Set[fy] = fx;
	return 1;
}

int n, m;
int mat[MAXN][MAXN];
const int dx[] = { -1, -1, -1, 0, 1, 1, 1, 0 };
const int dy[] = { -1, 0, 1, 1, 1, 0, -1, -1 };
int id(int x, int y) {
	return x * m + y;
}
bool isok(int x, int y) {
	return x >= 0 && x < n && y >= 0 && y < m;
}

int main() {
	while (scanf_s("%d %d", &n, &m) != EOF) {
		for (int i = 0; i < n; ++i) {
			for (int j = 0; j < m; ++j) {
				scanf_s("%d", &mat[i][j]);
			}
		}
		init_set();
		for (int i = 0; i < n; ++i) {
			for (int j = 0; j < m; ++j) {
				if (mat[i][j] == 0) continue;
				for (int d = 0; d < 8; ++d) {
					int ii = i + dx[d];
					int jj = j + dy[d];
					if (isok(ii, jj) && mat[ii][jj] != 0) {
						union_set(id(i, j), id(ii, jj));
					}
				}
			}
		}
		int ans = 0;
		for (int i = 0; i < n; ++i) {
			for (int j = 0; j < m; ++j) {
				if (mat[i][j] != 0) {
					ans = max(ans, -Set[id(i, j)]);
				}
			}
		}
		printf("%d\n", ans);
	}
	return 0;
}