Submission #1208569

#	Time	Username	Problem	Language	Result	Execution time	Memory
1208569		timeflew	Mecho (IOI09_mecho)	C++20	23 / 100	1096 ms	16388 KiB

mecho

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

const int MAX_N = 805;
int n, s;
char c[MAX_N][MAX_N];
vector<vector<int>> vis(MAX_N, vector<int>(MAX_N, INT_MAX)); // bee_time
pair<int, int> st, ed; // Start 'M' and end 'D' positions
int dx[4] = {0, 0, 1, -1};
int dy[4] = {1, -1, 0, 0};

// BFS to compute when bees reach each cell
void compute_bee_times() {
    queue<pair<int, int>> q;
    for (int i = 0; i < n; i++) {
        for (int j = 0; j < n; j++) {
            if (c[i][j] == 'H') {
                q.push({i, j});
                vis[i][j] = 0;
            }
        }
    }
    while (!q.empty()) {
        auto [x, y] = q.front();
        q.pop();
        for (int d = 0; d < 4; d++) {
            int nx = x + dx[d];
            int ny = y + dy[d];
            if (nx >= 0 && nx < n && ny >= 0 && ny < n && 
                (c[nx][ny] == 'G' || c[nx][ny] == 'M') && vis[nx][ny] == INT_MAX) {
                vis[nx][ny] = vis[x][y] + 1;
                q.push({nx, ny});
            }
        }
    }
}

// Check if Mecho can reach 'D' starting at minute k+1
bool check(int k) {
    if (vis[st.first][st.second] <= k) return false; // Caught at 'M'

    vector<pair<int, int>> current_positions;
    queue<array<int, 3>> q; // (x, y, dist)
    vector<vector<bool>> local_vis(n, vector<bool>(n, false));
    q.push({st.first, st.second, 0});
    local_vis[st.first][st.second] = true;

    // Positions reachable at the end of minute k+1
    while (!q.empty()) {
        auto [x, y, dist] = q.front();
        q.pop();
        if (dist > s) continue;
        if (vis[x][y] > k + 1) {
            current_positions.push_back({x, y});
        }
        for (int d = 0; d < 4; d++) {
            int nx = x + dx[d];
            int ny = y + dy[d];
            if (nx >= 0 && nx < n && ny >= 0 && ny < n && 
                c[nx][ny] != 'T' && c[nx][ny] != 'H' && !local_vis[nx][ny]) {
                local_vis[nx][ny] = true;
                q.push({nx, ny, dist + 1});
            }
        }
    }

    int t = k + 1;
    while (!current_positions.empty()) {
        if (find(current_positions.begin(), current_positions.end(), ed) != 
            current_positions.end()) {
            return true;
        }
        t++;
        vector<pair<int, int>> next_positions;
        q = queue<array<int, 3>>();
        local_vis.assign(n, vector<bool>(n, false));
        
        // Start BFS from all current positions
        for (auto [x, y] : current_positions) {
            if (!local_vis[x][y]) {
                q.push({x, y, 0});
                local_vis[x][y] = true;
            }
        }
        
        // Positions reachable by the end of minute t
        while (!q.empty()) {
            auto [x, y, dist] = q.front();
            q.pop();
            if (dist > s) continue;
            if (vis[x][y] > t) {
                next_positions.push_back({x, y});
            }
            for (int d = 0; d < 4; d++) {
                int nx = x + dx[d];
                int ny = y + dy[d];
                if (nx >= 0 && nx < n && ny >= 0 && ny < n && 
                    c[nx][ny] != 'T' && c[nx][ny] != 'H' && !local_vis[nx][ny]) {
                    local_vis[nx][ny] = true;
                    q.push({nx, ny, dist + 1});
                }
            }
        }
        current_positions = next_positions;
    }
    return false;
}

int main() {
    ios::sync_with_stdio(false);
    cin.tie(nullptr);
    
    cin >> n >> s;
    for (int i = 0; i < n; i++) {
        for (int j = 0; j < n; j++) {
            cin >> c[i][j];
            if (c[i][j] == 'M') st = {i, j};
            if (c[i][j] == 'D') ed = {i, j};
        }
    }

    compute_bee_times();

    int lo = 0, hi = n * n; // Maximum reasonable k
    int ans = -1;
    while (lo <= hi) {
        int mid = lo + (hi - lo) / 2;
        if (check(mid)) {
            ans = mid;
            lo = mid + 1;
        } else {
            hi = mid - 1;
        }
    }
    cout << ans << "\n";
    return 0;
}

• Subtask 1: 23 / 100