Submission #1056845

#	Time	Username	Problem	Language	Result	Execution time	Memory
1056845		TimAni	Mobile (BOI12_mobile)	C++17	0 / 100	1072 ms	31572 KiB

mobile

// time-limit: 3000

#include <bits/stdc++.h>

using namespace std;
using ll = long long;

struct point {
    double x{}, y{};
};

void solve() {
    int n;
    double L; 
    cin >> n >> L;
    vector<point> p(n);
    for(int i = 0; i < n; i++) {
        cin >> p[i].x >> p[i].y;
    }
    auto doubleersection = [&](const point& A, double R) -> array<double, 2> {
        // y^2 = 0, 
        // y^2 = R^2 - (x - A.x)^2 + 2yA.y - A.y ^ 2;
        // R^2 - (x - A.x) ^ 2 - A.y^2 = 0
        // R^2 - x^2 + 2xA.x - A.x^2 - A.y^2 = 0
        // x^2 - 2xA.x + A.x^2 + A.y^2 - R^2 = 0
        // D = -4A.y^2 + 4R^2
        if(abs(A.y) > R) {
            return {1.0 + L, -1.};
        }
        double D = 4. * (R * R - A.y * A.y);
        double x1 = (2. * A.x - sqrt(D)) / 2.;
        double x2 = (2. * A.x + sqrt(D)) / 2.;
        if(x2 < x1) swap(x1, x2);
        if(x1 < 0) x1 = 0; 
        if(x2 > L) x2 = L;
        return {x1, x2};
    };

    auto good = [&](double R) {
        vector<array<double, 2>> segs(n);
        for(double i = 0; i < n; i++) {
            segs[i] = doubleersection(p[i], R);
        }
        sort(segs.begin(), segs.end());
        double l = segs[0][0], r = segs[0][1];
        int i = 0;
        while(i + 1 < n && segs[i + 1][0] < r) {
            r = max(segs[i + 1][1], r);
            i++;
        }
        return r >= L && l <= 0;
    };

    double l = -1, r = 1;
    while(!good(r)) r *= 2;
    for(int i = 0; i < 100; i++) {
        double R = (r + l) / 2;
        if(good(R)) {
            r = R;
        }
        else {
            l = R;
        }
    }
    cout << fixed << setprecision(6) << r << endl;
}

int main() {
    cin.tie(0)->sync_with_stdio(0);
    double T = 1;
    //cin >> T;
    while(T--) solve();
    return 0;
}

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