bzoj3203

凸包+三分

想做这道题很久了

暴力的想法就是枚举，但是这样t了

然后发现列出来的式子满足三分

那么整数三分最大值

维护凸包的时候想了很长时间，觉得用斜率判很反人类

叉积是个好东西

#include<bits/stdc++.h>
using namespace std;
const int N = 1e5 + 5;
struct data {
    double x, y;
    data() {}
    data(double _, double __) : x(_), y(__) {}
    friend data operator - (const data &a, const data &b) {
        return data(a.x - b.x, a.y - b.y);
    }
    friend double operator * (const data &a, const data &b) {
        return a.x * b.y - a.y * b.x;
    }
} st[N];
int n, top;
double d, Ans, sum;
double cal(const data &a, const data &b) {
    return (a.y - b.y) / (a.x - b.x);
}
int main()
{
    scanf("%d%lf", &n, &d);
    for(int i = 1; i <= n; ++i)
    {
        double x, y, ans = 0;
        data t;
        scanf("%lf%lf", &x, &y);
        t.x = (double)i * d;
        t.y = sum;
        while(top > 1 && (t - st[top]) * (st[top] - st[top - 1]) >= 0) --top;
        st[++top] = t;
        sum += x;
        t.y += x;
        t.x += y;
        int l = 1, r = top;
        while(r - l >= 3) 
        {
            int L = (l * 2 + r) / 3, R = (l + 2 * r) / 3;
            if(cal(st[L], t) > cal(t, st[R])) r = R;
            else l = L;
        }
        for(int j = l; j <= r; ++j) ans = max(ans, cal(t, st[j]));
        Ans += ans;
    }
    printf("%.0f
", Ans);
    return 0;
}

cf上紫了!