BZOJ 1597 土地购买

#include<iostream>
#include<cstdio>
#include<cstring>
#include<algorithm>
#define maxn 50050
#define inf 0x7f7f7f7f7f7f7f7fLL
using namespace std;
long long n,s[maxn],top=0,f[maxn],g[maxn],a[maxn],b[maxn],l,r,q[maxn];
struct pnt
{
    long long x,y;
}p[maxn];
bool cmp(pnt x,pnt y)
{
    if (x.x!=y.x) return x.x<y.x;
    return x.y<y.y;
}
double k(long long x,long long y)
{
    if (b[x]==b[y]) return inf;
    return (double)(g[x]-g[y])/(b[x]-b[y]);
}
int main()
{
    scanf("%lld",&n);
    for (long long i=1;i<=n;i++) scanf("%lld%lld",&p[i].x,&p[i].y);
    sort(p+1,p+n+1,cmp);
    for (long long i=1;i<=n;i++)
    {
        while ((p[i].y>=p[s[top]].y) && (top)) top--;
        s[++top]=i;
    }
    for (long long i=1;i<=top;i++) a[i]=p[s[i]].x,b[i]=p[s[i]].y;
    f[1]=a[1]*b[1];l=r=1;q[l]=1;
    for (long long i=2;i<=top;i++)
    {
        g[i]=f[i-1];
        while ((r-l>0) && (k(q[r-1],q[r])<k(q[r],i))) r--;
        q[++r]=i;
        while ((r-l>0) && (k(q[l],q[l+1])>=-a[i])) l++;
        f[i]=g[q[l]]+a[i]*b[q[l]];
    }
    printf("%lld\n",f[top]);
    return 0;
}