[日常摸鱼]bzoj1007[HNOI2008]水平可见直线-半平面交（对偶转凸包）

#include<cstdio>
#include<algorithm>
const int N=50005;
inline int read()
{
    int s=0,f=1;char c=getchar();
    while(c<‘0‘||c>‘9‘){if(c==‘-‘)f=-1;c=getchar();}
    while(c>=‘0‘&&c<=‘9‘){s=s*10+c-‘0‘;c=getchar();}
    return s*f;
}
struct P{int x,y,i;}p[N];
int n,r,s[N];
bool cmp(P a,P b){return a.x<b.x||(a.x==b.x&&a.y<b.y);}
P operator-(P a,P b){return (P){a.x-b.x,a.y-b.y};}
long long cross(P a,P b){return 1ll*a.x*b.y-1ll*a.y*b.x;}
long long check(P a,P b,P c){return cross(b-a,c-a);} 
int main()
{
    n=read();int i;
    for(i=1;i<=n;i++){p[i].x=read(),p[i].y=-read(),p[i].i=i;}std::sort(p+1,p+n+1,cmp);
    for(i=1;i<=n;i++)
    {
        while(r>1&&check(p[s[r-1]],p[s[r]],p[i])<=0)--r;s[++r]=i;
    }
    for(i=1;i<=r;i++)s[i]=p[s[i]].i;std::sort(s+1,s+r+1);
    for(i=1;i<=r;i++)printf("%d ",s[i]);
    return 0;
}