[CSES] Counting Rooms

ll dx[4] = {1, -1, 0, 0};
ll dy[4] = {0, 0, 1, -1};
void solve()
{
    ll n, m;
    cin >> n >> m;

    vector<string> s(n);
    rep(i, n) cin >> s[i];

    vvll vis(n + 1, vll(m + 1, 0));
    function<void(ll, ll)> dfs = [&](ll x, ll y)
    {
        vis[x][y] = 1;
        rep(k, 4)
        {
            ll i = x + dx[k], j = y + dy[k];
            if (i >= 0 && i < n && j >= 0 && j < m && s[i][j] == '.' && !vis[i][j])
                dfs(i, j);
        }
    };

    ll cnt = 0LL;
    rep(i, n)
    {
        rep(j, m)
        {
            if (s[i][j] == '.' && !vis[i][j])
            {
                dfs(i, j);
                cnt++;
            }
        }
    }
    cout << cnt << "\n";
}