제출 #629702

#	제출 시각	아이디	문제	언어	결과	실행 시간	메모리
629702		54skyxenon	Mecho (IOI09_mecho)	C++17	100 / 100	439 ms	6600 KiB

mecho

// https://oj.uz/problem/view/IOI09_mecho
 
#include <bits/stdc++.h>
using namespace std;

#define pii pair<int, int>
#define MAX_N 800

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

int dx[4] = {-1, 0, 1, 0};
int dy[4] = {0, -1, 0, 1};

int mechox, mechoy, home_x, home_y;
vector<vector<bool>> bees_visited(MAX_N, vector<bool>(MAX_N));
vector<vector<int>> bees_time(MAX_N, vector<int>(MAX_N));
 
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;
}

// can Mecho eat for the given amount of time and still make it?
bool ok(int eating_time) {
    vector<vector<bool>> mecho_visited(n, vector<bool>(n));
    vector<vector<int>> mecho_time(n, vector<int>(n));
        
    // move Mecho
    queue<pii> q;
    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;
                mecho_time[nx][ny] = mecho_time[x][y] + 1;
                q.push({nx, ny});
            }
        }
    }

    // check if mecho reached a node surrounding his cave before the bees
    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]) {
            return true;
        }
    }
    
    return false;
}

int bs(int lo, int hi) {
    if (lo > hi) {
        return -1;
    }

    int mid = (lo + hi) / 2;

    if (!ok(mid)) {
        return bs(lo, mid - 1);
    }
    else if (mid < hi && ok(mid + 1)) {
        return bs(mid + 1, hi);
    }
    else {
        return mid;
    }
}
 
int main() {
    cin >> n >> s;
    for (int i = 0; i < n; i++) {
        cin >> field[i];
    }
    
    // find x and y coordinates for for Mecho, the bees and the cave
    vector<pii> hives;
    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;
            }
        }
    }

    // move bees
    queue<pii> q;
    for (auto [hx, hy] : hives) {
        q.push({hx, hy});
        bees_visited[hx][hy] = 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;
            }
        }
    }
 
    // binary search on waiting time
    cout << bs(0, n * n) << "\n";
}

• Subtask 1: 100 / 100