Submission #1056808

#	Time	Username	Problem	Language	Result	Execution time	Memory
1056808		TimAni	Mobile (BOI12_mobile)	C++17	0 / 100	1102 ms	39520 KiB

mobile

// time-limit: 3000

#include <bits/stdc++.h>

using namespace std;
using ll = long long;

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

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

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

    long double l = 0, r = hypot(2e9, 1e9);
    for(int i = 0; i < 50; i++) {
        long 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);
    int T = 1;
    //cin >> T;
    while(T--) solve();
    return 0;
}

Compilation message (stderr)

mobile.cpp: In lambda function:
mobile.cpp:26:23: warning: narrowing conversion of '(L + 1)' from 'int' to 'long double' [-Wnarrowing]
   26 |             return {L + 1, -1};
      |                     ~~^~~
mobile.cpp:31:9: warning: this 'if' clause does not guard... [-Wmisleading-indentation]
   31 |         if(x1 < 0) x1 = 0; if(x2 > L) x2 = L;
      |         ^~
mobile.cpp:31:28: note: ...this statement, but the latter is misleadingly indented as if it were guarded by the 'if'
   31 |         if(x1 < 0) x1 = 0; if(x2 > L) x2 = L;
      |                            ^~

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