Submission #1011173

#	Time	Username	Problem	Language	Result	Execution time	Memory
1011173		vjudge1	Mobile (BOI12_mobile)	C++17	0 / 100	312 ms	16636 KiB

mobile

#include <bits/stdc++.h>
using i64 = long long;

struct Point {
    double x, y;
};

double dist(const Point &a, const Point &b) {
    double dx = a.x - b.x;
    double dy = a.y - b.y;
    return std::sqrt(dx * dx + dy * dy);
}

double insec(const Point &a, const Point &b) {
    double t1 = b.x * b.x + b.y * b.y - a.x * a.x - a.y * a.y;
    double t2 = b.x - a.x;
    return t1 / t2 / 2;
}

int main() {
    std::ios::sync_with_stdio(false);
    std::cin.tie(nullptr);

    int n;
    double l;
    std::cin >> n >> l;

    std::deque<Point> pt;
    for (int i = 0; i < n; i++) {
        double x, y;
        std::cin >> x >> y;
        y = std::abs(y);

        if (pt.size() && pt.back().x == x) {
            if (pt.back().y <= y) {
                continue;
            } else {
                pt.pop_back();
            }
        }
        pt.push_back({x, y});
    }

    while ((int)pt.size() > 1 && insec(pt[0], pt[1]) < 0) {
        pt.pop_front();
    }
    while ((int)pt.size() > 1 && insec(pt[pt.size() - 1], pt[pt.size() - 2]) > l) {
        pt.pop_back();
    }

    double ans = 0;
    Point lt{0, 0}, rt{l, 0};
    for (int i = 0; i < (int)pt.size(); i++) {
        if (i > 0) {
            lt.x = insec(pt[i], pt[i - 1]);
        }
        if (i + 1 < (int)pt.size()) {
            rt.x = insec(pt[i], pt[i + 1]);
        }
        if (lt.x < 0 || lt.x > l || rt.x < 0 || rt.x > l) {
            continue;
        }
        ans = std::max({ans, dist(lt, pt[i]), dist(pt[i], rt)});
    }
    std::cout << std::fixed << std::setprecision(6) << ans << "\n";

    return 0;
}

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