codeforces 679 C

#include <iostream>
#include <fstream>
#include <set>
#include <map>
#include <string>
#include <vector>
#include <bitset>
#include <algorithm>
#include <cstring>
#include <cstdlib>
#include <cmath>
#include <cassert>
#include <queue>

typedef long long ll;
typedef long double ld;

using namespace std;
const int MAXN = 510;

int n, k;
int arr[MAXN][MAXN];
int was[MAXN][MAXN];
int cl[MAXN][MAXN];
int nc[MAXN * MAXN];
int lst[MAXN * MAXN];
int tm;
int cc;
int cnt[MAXN * MAXN];
int dx[] = {0, 0, 1, -1};
int dy[] = {1, -1, 0, 0};

void dfs(int x, int y, int c) {
was[x][y] = 1;
cl[x][y] = c;
++cnt[c];
for (int i = 0; i < 4; ++i) {
int nx = x + dx[i];
int ny = y + dy[i];
if (!arr[nx][ny] && !was[nx][ny])
dfs(nx, ny, c);
}
}

int add(int x, int y) {
if (arr[x][y])
return 0;
if (lst[cl[x][y]] == tm)
return 0;
lst[cl[x][y]] = tm;
return cnt[cl[x][y]] - nc[cl[x][y]];
}

int main() {
scanf("%d%d", &n, &k);
for (int i = 0; i <= n + 1; ++i)
for (int j = 0; j <= n + 1; ++j)
arr[i][j] = 1;
for (int i = 1; i <= n; ++i)
for (int j = 1; j <= n; ++j) {
char c;
scanf(" %c", &c);
if (c == '.')
arr[i][j] = 0;
else
arr[i][j] = 1;
}
cc = 0;
for (int i = 1; i <= n; ++i)
for (int j = 1; j <= n; ++j) {
if (!arr[i][j] && !was[i][j]) {
dfs(i, j, cc++);
}
}
int ans = 0;
for (int i = 0; i < cc; ++i)
ans = max(ans, cnt[i]);
for (int i = 1; i <= n - k + 1; ++i) {
memset(nc, 0, sizeof(nc[0]) * cc);
for (int j = i; j < i + k; ++j)
for (int j1 = 1; j1 <= k; ++j1)
if (!arr[j][j1])
++nc[cl[j][j1]];
for (int j = 1; j <= n - k + 1; ++j) {
++tm;
int now = k * k;
for (int i1 = i; i1 < i + k; ++i1)
now += add(i1, j - 1), now += add(i1, j + k);
for (int j1 = j; j1 < j + k; ++j1)
now += add(i - 1, j1), now += add(i + k, j1);
ans = max(ans, now);
for (int i1 = i; i1 < i + k; ++i1)
if (!arr[i1][j])
--nc[cl[i1][j]];
for (int i1 = i; i1 < i + k; ++i1)
if (!arr[i1][j + k])
++nc[cl[i1][j + k]];
}
}
cout << ans << "\n";
return 0;
}