Submission #1294514

#	Time	Username	Problem	Language	Result	Execution time	Memory
1294514		nghiafl1	Mecho (IOI09_mecho)	C++20	100 / 100	210 ms	6600 KiB

mecho

#include <bits/stdc++.h>
using namespace std;

const int MAX_N = 800;
vector<string> field(MAX_N);
int n, s;

bool valid_sq(int x, int y) {
    return x >= 0 && x < n && y >= 0 && y < n &&
           (field[x][y] == 'G' || field[x][y] == 'M');
}

bool mecho_reached(int mecho_dis, int bees_dis) { return mecho_dis / s < bees_dis; }

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;
            }
        }
    }

    int dx[] = {-1, 0, 1, 0};
    int dy[] = {0, -1, 0, 1};

    // binary search on waiting time
    int l = 0;
    int r = n * n;
    vector<vector<bool>> bees_visited(n, vector<bool>(n));
        vector<vector<int>> bees_time(n, vector<int>(n));
        queue<pair<int, int>> q;
    // 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;
                }
            }
        }

    while (l <= r) {
        
        vector<vector<bool>> mecho_visited(n, vector<bool>(n));
        
        vector<vector<int>> mecho_time(n, vector<int>(n));
        

        int eating_time = (l + r) / 2;

        

        // 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.
                 * subtract 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_reached(mecho_time[x][y] + 1,
                                  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 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_reached(mecho_time[nx][ny], bees_time[nx][ny] - eating_time) &&
                mecho_visited[nx][ny]) {
                reached = true;
            }
        }
        if (reached) {
            l = eating_time + 1;
        } else {
            r = eating_time - 1;
        }
    }

    cout << l - 1 << '\n';
}

• Subtask 1: 100 / 100