bzoj1597 [Usaco2008 Mar]土地购买——斜率优化DP

题目：https://www.lydsy.com/JudgeOnline/problem.php?id=1597

就是斜率优化水题…

然而WA了十几遍，正负号处理真让人心累…

还是该负就负，别乱换了…

代码如下：

#include<iostream>
#include<cstdio>
#include<cstring>
#include<algorithm>
using namespace std;
typedef long long ll;
int const maxn=;
int n,cnt,q[maxn],h,t;
ll f[maxn],x[maxn],y[maxn];
struct N{ll a,b;}p[maxn];
ll rd()
{
    ll ret=,f=;char ch=getchar();
    while(ch<''||ch>''){if(ch=='-')f=-; ch=getchar();}
    while(ch>=''&&ch<='')ret=ret*+ch-'',ch=getchar();
    return ret*f;
}
//bool cmp(N x,N y){return x.a==y.a?x.b>y.b:x.a<y.a;}
bool cmp(N x,N y){return x.a==y.a?x.b>y.b:x.a<y.a;}
double slp(int a,int b){return (double)(f[b]-f[a])/(double)(y[a+]-y[b+]);}
int main()
{
    n=rd();
    for(int i=;i<=n;i++)p[i].a=rd(),p[i].b=rd();
    sort(p+,p+n+,cmp);
    for(int i=;i<=n;i++)
    {
//        if(p[i].a==p[i-1].a)continue;
//        x[++cnt]=p[i].a; y[cnt]=p[i].b;
        while (cnt &&  p[i].b>=y[cnt]) cnt--;
        x[++cnt]=p[i].a; y[cnt]=p[i].b;
    }
//    h=1;t=1;//
    for(int i=;i<=cnt;i++)
    {
        while(h<t&&slp(q[h],q[h+])<x[i])h++;//<
        f[i]=f[q[h]]+x[i]*y[q[h]+];
        while(h<t&&slp(q[t-],q[t])>slp(q[t],i))t--;
        q[++t]=i;
    }
    printf("%lld",f[cnt]);
    return ;
}