Competitive Programming

#include <bits/stdc++.h>
#define INF 0x3F3F3F3F
using namespace std;

int cx[4] = {1, -1, 0, 0};
int cy[4] = {0, 0, 1, -1};

char mainMap[2000][2000];
bool reachable[2000][2000];
int beeDistance[2000][2000];

int n, s,dx, dy, mx, my;

bool test(int delay)
{
    if (delay * s >= beeDistance[mx][my]) return false;
    memset(reachable, 0, sizeof(reachable));
    deque<pair<int, pair<int, int> > > q;
    q.push_back(make_pair(delay * s, make_pair(mx, my)));
    reachable[mx][my] = true;
    while (!q.empty())
    {
        int distance = q.front().first;
        int x = q.front().second.first;
        int y = q.front().second.second;
        q.pop_front();
        if (mainMap[x][y] == 'D') return true;
        for (int c = 0; c < 4; c++)
        {
            int nx = x + cx[c];
            int ny = y + cy[c];
            if (nx < 0 || nx >= n || ny < 0 || ny >= n || mainMap[nx][ny] == 'T' || (distance + 1) >= beeDistance[nx][ny] || reachable[nx][ny]) continue;
            q.push_back(make_pair(distance + 1, make_pair(nx, ny)));
            reachable[nx][ny] = true;
        }
    }
    return false;
}
int main()
{
    int cases;
    cin>>cases;
    while(cases--)
    {
    cin>>n>>s;
    deque<pair<int, int> > bq;
    memset(beeDistance, -1, sizeof(beeDistance));

    for (int i = 0; i < n; i++)
    {
        cin >> ws;
        for (int j = 0; j < n; j++)
        {
            cin >> mainMap[i][j];
            if (mainMap[i][j] == 'H')
            {
                bq.push_back(make_pair(i, j));
                beeDistance[i][j] = 0;
            }
            else if (mainMap[i][j] == 'M')
            {
                mx = i;
                my = j;
                mainMap[i][j] = 'G';
            }
            else if (mainMap[i][j] == 'D')
            {
                dx = i;
                dy = j;
            }
        }
    }
    //Tiempo de las abejas llegar a otro sitio
    while (!bq.empty())
    {
        int x = bq.front().first;
        int y = bq.front().second;
        bq.pop_front();
        for (int c = 0; c < 4; c++)
        {
            int nx = x + cx[c];
            int ny = y + cy[c];
            if (nx < 0 || nx >= n || ny < 0 || ny >= n || mainMap[nx][ny] != 'G' || beeDistance[nx][ny] != -1)  continue;
            beeDistance[nx][ny] = beeDistance[x][y] + s;
            bq.push_back(make_pair(nx, ny));
        }
    }
    //A su casa INF tiempo
    beeDistance[dx][dy] = INF;

    int low = -1, high = n * n;
    while (high - low > 1)
    {
        int mid = (low + high)/2;
        if (test(mid))
            low = mid;
        else
            high = mid;
    }
    cout << low << '\n';
    }
    return 0;
}