Submission #1063885

#	Time	Username	Problem	Language	Result	Execution time	Memory
1063885		daviedu	Mobile (BOI12_mobile)	C++17	0 / 100	157 ms	16724 KiB

mobile

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

#define fastio ios_base::sync_with_stdio(0); cin.tie(0)
#define all(a) a.begin(), a.end()
#define rall(a) a.rbegin(), a.rend()
#define ll long long
#define ld long double

struct P{
    ll x, y;
};

void dbg_out() { cerr << endl; }
template <typename H, typename... T>
void dbg_out(H h, T... t) { cerr << ' ' << h; dbg_out(t...); }
#define dbg(...) { cerr << #__VA_ARGS__ << ':'; dbg_out(__VA_ARGS__); }

ld border(ld a, ld b, ld x, ld y){
    return (a*a + b*b - x*x - y*y) / (2*b - 2*x);
}

ld dist(ld x, ld y, ld point){
    return sqrtl(y*y + (point-x)*(point-x));
}

signed main(){
    fastio;
    int n, r;
    cin >> n >> r;
    stack<pair<int, int>> st;
    int a, b, x, y;
    int last_x=-1;
    for (int i=0; i<n; i++){
        cin >> a >> b;
        if (b < 0) b = -b;
        if (a == last_x) continue;
        last_x = a;

        while ((int)st.size() > 1){
            tie(x, y) = st.top();
            st.pop();

            // if prev is not a useless point, put it back and stop
            if (border(x, y, a, b) > border(st.top().first, st.top().second, a, b)){
                st.push({x, y});
                break;
            }
        }

        st.push({a, b});
    }

    ld ans=0;
    ld right=r;
    while ((int)st.size() > 1){
        tie(a, b) = st.top();
        st.pop();
        tie(x, y) = st.top();
        ld left = border(x, y, a, b);
        if (left >= right) continue;
        if (right >= 0 && right <= r) ans = max(ans, dist(a, b, right));
        if (left >= 0 && left <= r) ans = max(ans, dist(a, b, left));
        right = left;
    }
    tie(a, b) = st.top();
    if (right >= 0) {
        ans = max(ans, dist(a, b, right));
        ans = max(ans, dist(a, b, 0));
    }
    cout << fixed << setprecision(13);
    cout << ans << '\n';
}

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