BFS - C

#include <iostream>
#include <queue>
#include <algorithm>

using namespace std;

const int N = 1001;
const int INF = 1e9 + 1;
const int dx[8] = {1, -1, 0, 0, 1, 1, -1, -1};
const int dy[8] = {0, 0, 1, -1, 1, -1, 1, -1};

int n, m, dist[N][N];
char a[N][N];

int main()
{
    cin >> n >> m;
    for (int i = 0; i < n; i++)
        for (int j = 0; j < m; j++) {
            cin >> a[i][j];
            dist[i][j] = INF;
        }
    queue<pair<int, int>> q;
    int k;
    cin >> k;
    for (int i = 0; i < k; i++) {
        int x, y;
        cin >> x >> y;
        x--;
        y--;
        dist[x][y] = 0;
        q.push({x, y});
    }

    while (!q.empty()) {
        auto [x, y] = q.front();
        q.pop();
        ///check neighbours
        for (int d = 0; d < 8; d++) {
            int ax = x + dx[d];
            int ay = y + dy[d];
            if (ax >= 0 && ax < n && ay >= 0 && ay < m && a[ax][ay] == '.' && dist[ax][ay] == INF) {
                dist[ax][ay] = dist[x][y] + 1;
                q.push({ax, ay});
            }
        }
    }

    int mx = -1;
    for (int i = 0; i < n; i++)
        for (int j = 0; j < m; j++)
            if (dist[i][j] != INF)
                mx = max(mx, dist[i][j]);

    vector<pair<int, int>> ans;
    for (int i = 0; i < n; i++)
        for (int j = 0; j < m; j++)
            if (dist[i][j] == mx)
                ans.push_back({i, j});
    cout << ans.size() << "\n";
    for (auto [x, y] : ans)
        cout << x + 1 << ' ' << y + 1 << "\n";

    return 0;
}