3203: [Sdoi2013]保护出题人 凸包+三分

膜IOI爷的题解
凸包上三分。。

#include
#include
#define N 100105
using namespace std;
int n,top,stack[N];
double d;
double sum[N],x[N];
struct node {double x,y;} a[N];
double cross(node a,node b,node c)
{
    return (a.x-c.x)*(b.y-c.y)-(b.x-c.x)*(a.y-c.y);
}
double slop(node a,node b)
{
    return (a.y-b.y)/(a.x-b.x);
}
double find(node t)
{
    int l=1,r=top,lmid,rmid;
    while (r-l>=3)
    {
        lmid=l+(r-l)/3;
        rmid=r-(r-l)/3;
        double K1=slop(a[stack[lmid]],t),K2=slop(a[stack[rmid]],t);
        if (K1else r=rmid;
    }
    double ans=0.0;
    for (int i=l;i<=r;i++) ans=max(ans,slop(a[stack[i]],t));
    return ans;
}
int main()
{
    scanf("%d%lf",&n,&d);
    for (int i=1;i<=n;i++)
    {
        scanf("%lf%lf",&sum[i],&x[i]);
        sum[i]+=sum[i-1]; 
        a[i]=(node){i*d,sum[i-1]};
    }
    double ans=0;
    for (int i=1;i<=n;i++)
    {
        while (top>1&&cross(a[i],a[stack[top]],a[stack[top-1]])>=0) top--;
        stack[++top]=i;
        node q=(node){x[i]+i*d,sum[i]};
        ans+=find(q);
    }
    printf("%.0lf\n",ans);
    return 0;
}