bfs,o2： 128 ms

#include <bits/stdc++.h>
using namespace std;
const int N = 1010;
char g[N][N];
bool st[N][N];
int n;
int ans = 0;
int dx[] = {0, 0, -1, 1}, dy[] = {1, -1, 0, 0};
typedef pair<int,int> PII;

void bfs(int x, int y, int &bound, int &total)
{
    total++;

    st[x][y] = true;
    PII start = {x, y};
    queue<PII> q;
    q.push(start);
    while(!q.empty()) {
        auto tt = q.front();
        q.pop();
        bool is_bound = false;
        for(int i = 0; i < 4; i++) {
            int tx = tt.first + dx[i], ty = tt.second + dy[i];
            if(tx < 0 || tx >= n || ty < 0 || ty >= n) continue;
            if(st[tx][ty]) continue;
            if(g[tx][ty] == '.') {
                is_bound = true;
                continue;
            }
            q.push({tx, ty});
            total++;
            st[tx][ty] = true;
        }
        if(is_bound) bound++;
    }
}

int main()
{
    ios::sync_with_stdio(false);
    cin.tie(nullptr);
    cin >> n; 
    for(int i = 0 ; i < n ; i++) {
        cin >> g[i];
    }

    for(int i = 0 ; i < n;  i++) {
        for(int j = 0 ; j < n ; j++) {
            if(!st[i][j] && g[i][j] == '#') {
                int bound = 0, total = 0;
                bfs(i, j, bound, total);
                if(bound == total) ans++;
            }
        }
    }
    cout << ans << endl;
    return 0;   
}

dfs,O2:81 ms

#include <bits/stdc++.h>
using namespace std;
const int N = 1010;
char g[N][N];
bool st[N][N];
int n;
int ans = 0;
int dx[] = {0, 0, -1, 1}, dy[] = {1, -1, 0, 0};

void dfs(int tx, int ty, int &bound, int &total)
{
    st[tx][ty] = true;
    bool is_bound = false;
    total++;
    for(int i = 0; i < 4; i++) {
        int x = tx + dx[i];
        int y = ty + dy[i];
        if(x < 0 || x >= n || y < 0 || y >= n) continue;
        if(st[x][y]) continue;

        if(g[x][y] == '.') {
            is_bound = true;
            continue;
        }
        else {
            dfs(x, y, bound, total);
        }
    }
    if(is_bound) bound++;   
}

int main()
{
    ios::sync_with_stdio(false);
    cin.tie(nullptr);
    cin >> n; 
    for(int i = 0 ; i < n ; i++) {
        cin >> g[i];
    }

    for(int i = 0 ; i < n;  i++) {
        for(int j = 0 ; j < n ; j++) {
            if(!st[i][j] && g[i][j] == '#') {
                int bound = 0, total = 0;
                dfs(i, j, bound, total);
                if(bound == total) ans++;
            }
        }
    }
    cout << ans << endl;
    return 0;   
}