Submission #491399

#	Time	Username	Problem	Language	Result	Execution time	Memory
491399		nehasane	Mecho (IOI09_mecho)	C++14	53 / 100	436 ms	7720 KiB

mecho

//for an editorial and code in C++, visit my blog - https://nehasane.blogspot.com/2021/11/ioi-2009-mecho.html
#include <bits/stdc++.h>
using namespace std;
const int MAX_N = 800;
vector <string> field(MAX_N);
bool bees_visited[MAX_N][MAX_N], mecho_visited[MAX_N][MAX_N];
//mecho_time records the time taken for mecho to reach node[x][y].
//bees_time does the same for the bees
int mecho_time[MAX_N][MAX_N], bees_time[MAX_N][MAX_N];
queue <pair <int, int>> q;
int dx[] = {-1, 1, 0, 0};
int dy[] = {0, 0, -1, 1};
int n, s;
bool valid_sq(int x, int y){
    if (x >= 0 && x < n && y >= 0 && y < n && (field[x][y] == 'G' || field[x][y] == 'M'))
        return true;
    return false;
}
int main()
{
    cin >> n >> s;
    for (int i = 0; i < n; i++)
        cin >> field[i];
    vector <pair <int, int>> hives;
    int mechox, mechoy, home_x, home_y;
    //find x and y coordinates for for Mecho, the bees and the cave
    for (int i = 0; i < n; i++){
        for (int j = 0; j < n; j++){
            if (field[i][j] == 'M'){
                mechox = i;
                mechoy = j;
            }else if (field[i][j] == 'H'){
                hives.push_back({i, j});
            }else if (field[i][j] == 'D'){
                home_x = i;
                home_y = j;
            }
        }
    }
    //binary search on waiting time
    int l = 0, r = 2*n;
    while (l <= r){
        int eating_time = (l + r) / 2;
        memset(bees_visited, false, sizeof(bees_visited));
        memset(mecho_visited, false, sizeof(mecho_visited));
        memset(bees_time, 0, sizeof(bees_time));
        memset(mecho_time, 0, sizeof(mecho_time));
        //move bees
        for (auto i : hives){
            q.push({i.first, i.second});
            bees_visited[i.first][i.second] = true;
        }
        while (!q.empty()){
            int x = q.front().first, y = q.front().second;
            q.pop();
            for (int i = 0; i < 4; i++){
                int nx = x + dx[i], ny = y + dy[i];
                if (valid_sq(nx, ny) && !bees_visited[nx][ny]){
                    bees_time[nx][ny] = bees_time[x][y] + 1;    
                    q.push({nx, ny});
                    bees_visited[nx][ny] = true;
                }
            }
        }
        //move Mecho
        q.push({mechox, mechoy});
        mecho_visited[mechox][mechoy] = true;
        if (bees_time[mechox][mechoy] <= eating_time)
            q.pop();
        while (!q.empty()){
            int x = q.front().first, y = q.front().second;
            q.pop();
            for (int i = 0; i < 4; i++){
                int nx = x + dx[i], ny = y + dy[i];
                //check if mecho reaces node[x][y] before the bees
                /*divide the time mecho takes to reach a node by s, since mecho
                  walks s steps at a time.
                  substract the eating time from the time taken for the bees to 
                  reach the node, because that time was used by mecho for eating
                */
                if (valid_sq(nx, ny) && !mecho_visited[nx][ny] && 
                    (mecho_time[x][y] + 1) / s < bees_time[nx][ny] - eating_time){
                    mecho_visited[nx][ny] = true;
                    q.push({nx, ny});
                    mecho_time[nx][ny] = mecho_time[x][y] + 1;
                }
            }
        }
        //check if mecho reached a node surrounding his cave before the bees
        bool mecho_reached = false;
        for (int i = 0; i < 4; i++){
            int nx = home_x+ dx[i], ny = home_y + dy[i];
            if (valid_sq(nx, ny) && (mecho_time[nx][ny] / s) < bees_time[nx][ny] - eating_time
                && mecho_visited[nx][ny])
                mecho_reached = true;
        }
        if (mecho_reached)
            l = eating_time + 1;
        else
            r = eating_time - 1;
        }
    cout << l - 1 << '\n';
}

Compilation message (stderr)

mecho.cpp: In function 'int main()':
mecho.cpp:68:37: warning: 'mechoy' may be used uninitialized in this function [-Wmaybe-uninitialized]
   68 |         if (bees_time[mechox][mechoy] <= eating_time)
      |             ~~~~~~~~~~~~~~~~~~~~~~~~^
mecho.cpp:68:37: warning: 'mechox' may be used uninitialized in this function [-Wmaybe-uninitialized]
mecho.cpp:92:37: warning: 'home_y' may be used uninitialized in this function [-Wmaybe-uninitialized]
   92 |             int nx = home_x+ dx[i], ny = home_y + dy[i];
      |                                     ^~
mecho.cpp:92:17: warning: 'home_x' may be used uninitialized in this function [-Wmaybe-uninitialized]
   92 |             int nx = home_x+ dx[i], ny = home_y + dy[i];
      |                 ^~

• Subtask 1: 53 / 100