题解

沙发！

#include <bits/stdc++.h>
using namespace std;
#define read cin
#define written cout
vector<pair<int, int>> neighbors(int r, int c) {
	return {{r + 1, c}, {r - 1, c}, {r, c + 1}, {r, c - 1}};
}

int main() {
	int side_len;
	read >> side_len;
	vector<vector<int>> grid(side_len, vector<int>(side_len));
	for (int r = 0; r < side_len; r++) {
		for (int c = 0; c < side_len; c++) { read >> grid[r][c]; }
	}
	vector<vector<int>> regions(side_len, vector<int>(side_len));
	vector<vector<pair<int, int>>> region_cells;

	int one_biggest = 0;
	vector<vector<bool>> visited(side_len, vector<bool>(side_len));
	for (int r = 0; r < side_len; r++) {
		for (int c = 0; c < side_len; c++) {
			if (visited[r][c]) { continue; }

			int curr_region = region_cells.size();

			vector<pair<int, int>> contained;

			vector<pair<int, int>> frontier{{r, c}};
			visited[r][c] = true;
			while (!frontier.empty()) {
				pair<int, int> curr = frontier.back();
				frontier.pop_back();

				contained.push_back(curr);
				regions[curr.first][curr.second] = curr_region;

				for (const auto &[nr, nc] :
				     neighbors(curr.first, curr.second)) {
					if (0 <= nr && 0 <= nc && nr < side_len && nc < side_len &&
					    !visited[nr][nc] && grid[nr][nc] == grid[r][c]) {
						visited[nr][nc] = true;
						frontier.push_back({nr, nc});
					}
				}
			}
			one_biggest = std::max(one_biggest, (int)contained.size());
			region_cells.push_back(contained);
		}
	}
	vector<std::set<int>> adj_regions(region_cells.size());
	for (const vector<pair<int, int>> &reg : region_cells) {
		for (const auto &[r, c] : reg) {
			for (const auto &[nr, nc] : neighbors(r, c)) {
				if (0 <= nr && 0 <= nc && nr < side_len && nc < side_len &&
				    regions[nr][nc] != regions[r][c]) {
					adj_regions[regions[r][c]].insert(regions[nr][nc]);
				}
			}
		}
	}
	auto region_id = [&](int r) {
		return grid[region_cells[r][0].first][region_cells[r][0].second];
	};
	std::map<pair<int, int>, std::set<int>> seen;
	int two_biggest = one_biggest;
	for (int r1 = 0; r1 < region_cells.size(); r1++) {
		for (int r2 : adj_regions[r1]) {
			pair<int, int> valid{region_id(r1), region_id(r2)};
			if (valid.first > valid.second) {
				std::swap(valid.first, valid.second);
			}
			if (seen[valid].count(r1)) { continue; }
			int two_size = 0;
			vector<int> frontier{r1};
			vector<bool> curr_vis(region_cells.size());
			curr_vis[r1] = true;
			while (!frontier.empty()) {
				int curr = frontier.back();
				frontier.pop_back();
				two_size += region_cells[curr].size();
				seen[valid].insert(curr);
				for (int nr : adj_regions[curr]) {
					int nid = region_id(nr);
					if (!curr_vis[nr] &&
					    (valid.first == nid || valid.second == nid)) {
						curr_vis[nr] = true;
						frontier.push_back(nr);
					}
				}
			}
			two_biggest = max(two_biggest, two_size);
		}
	}

	written << one_biggest << '\n' << two_biggest << endl;
}