Revise paste USACO 2017 US Open Contest, Bronze - Problem...

#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);
}