Submission #1249014

#	Time	Username	Problem	Language	Result	Execution time	Memory
1249014		mirbek01	Mobile (BOI12_mobile)	C++20	15 / 100	649 ms	20172 KiB

mobile

#include <bits/stdc++.h>

using namespace std;

double dist(double x, double y, double a, double b) {
    return sqrt((x - a) * (x - a) + (y - b) * (y - b));
}

signed main() {
    int n;
    double L;
    cin >> n >> L;

    vector <double> x(n), y(n);
    for (int i = 0; i < n; i++) {
        cin >> x[i] >> y[i];
    }

    auto get_point = [&](int i, int j) {
        return ((x[i] * x[i] + y[i] * y[i]) - (x[j] * x[j] + y[j] * y[j])) / (2.0 * (x[i] - x[j]));
    };  

    deque <int> stk;
    for (int i = 0; i < n; i++) {
        if (!stk.empty() && x[stk.back()] == x[i]) {
            if (y[i] >= y[stk.back()]) {
                continue;
            } else {
                stk.pop_back();
            }
        }
        while (stk.size() > 1 && get_point(stk.back(), i) < get_point(stk[stk.size() - 2], stk.back())) stk.pop_back();
        stk.push_back(i);
    }

    while (stk.size() > 1 && get_point(stk[0], stk[1]) < 0) {
        stk.pop_front();
    }
    while (stk.size() > 1 && get_point(stk[stk.size() - 2], stk[stk.size() - 1]) > L) {
        stk.pop_back();
    }

    double ans = 0;
    for (int i = 0; i < stk.size(); i++) {
        double left = 0, right = L;
        if (i) {
            left = get_point(stk[i - 1], stk[i]);
        }
        if (i + 1 < stk.size()) {
            right = get_point(stk[i], stk[i + 1]);
        }
        if (left < 0 || right > L || left > L || right < 0) continue;
        ans = max(ans, dist(x[stk[i]], y[stk[i]], left, 0));
        ans = max(ans, dist(x[stk[i]], y[stk[i]], right, 0));
    }

    cout << fixed << setprecision(3) << ans << endl;
}

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