bzoj3203: [Sdoi2013]保护出题人 凸包+三分

/**************************************************************
    Problem: 3203
    User: wangyucheng
    Language: C++
    Result: Accepted
    Time:344 ms
    Memory:4396 kb
****************************************************************/
 
#include<iostream>
#include<cstdio>
#include<algorithm>
#include<cstring>
#include<cmath>
using namespace std;
#define N 100003
typedef long long ll;
int n;
double d,a[N],x[N],ans=0.0;
struct P{
    double x,y;
    P(double a=0.0,double b=0.0){
     x=a,y=b;   
    }   
    P operator-(P b){return P(x-b.x,y-b.y);}
    double operator*(P b){return x*b.y-y*b.x;}
}s[N];
double sp(P a,P b){
   return (a.y-b.y)/(a.x-b.x);  
}
int main(){
    scanf("%d",&n);
    scanf("%lf",&d);
    for(int i=1;i<=n;i++)scanf("%lf%lf",&a[i],&x[i]);
    double sum =0;
    int top=0;
    for(int i=1;i<=n;i++){
        P p=P(i*d,sum);sum+=a[i];
        while(top>1&&(p-s[top-1])*(s[top]-s[top-1])>=0)top--;
        s[++top]=p;
        p=P(x[i]+i*d,sum);
        int l=1,r=top,m1,m2;
        while(r-l>=3){
           m1=l+(r-l)/3;
           m2=r-(r-l)/3;
           double k1=sp(s[m1],p),k2=sp(s[m2],p);
           if(k1<k2)l=m1;else r=m2;  
        }
        double res=0;
        for(int j=l;j<=r;j++)res=max(res,sp(s[j],p));
        ans+=res;   
      
    }
    printf("%.0lf\n",ans);
}