USACO 2017 US Open Contest, Bronze - Problem 3. Modern Art

#include <bits/stdc++.h>
using namespace std;
ifstream fin("art.in");
ofstream fout("art.out");

const int nmax = 10;
int n;
int c[nmax + 1][nmax + 1];
struct BoundingBox {
    int xmin = 1e9, ymin = 1e9;
    int xmax = 0, ymax = 0;
} bb[nmax + 1];

void update(BoundingBox& bb, int x, int y) {
    bb.xmin = min(bb.xmin, x);
    bb.ymin = min(bb.ymin, y);
    bb.xmax = max(bb.xmax, x);
    bb.ymax = max(bb.ymax, y);
}

bool ok[nmax + 1];

int main() {
    fin >> n;
    for (int i = 1; i <= n; ++i) {
        for (int j = 1; j <= n; ++j) {
            char ch;
            fin >> ch;
            c[i][j] = ch - '0';
            if (c[i][j] == 0) {
                continue;
            }
            ok[c[i][j]] = true;
            update(bb[c[i][j]], i, j);
        }
    }
    for (int i = 1; i <= n; ++i) {
        for (int j = 1; j <= n; ++j) {
            if (c[i][j] == 0) {
                continue;
            }
            for (int k = 1; k <= 9; ++k) {
                if (k == c[i][j]) {
                    continue;
                }
                // vreau sa vad daca (i, j) este inclusa in bounding boxul culorii k
                if (bb[k].xmin <= i && i <= bb[k].xmax && bb[k].ymin <= j && j <= bb[k].ymax) {
                    // culoarea c[i][j] a fost pusa dupa culoarea k
                    ok[c[i][j]] = false;
                }
            }
        }
    }
    fout << count(ok + 1, ok + 11, true);
}