bzoj1007: [HNOI2008]水平可见直线

单调栈维护下凸包。

#include<cstdio>
#include<iostream>
#include<cstring>
#include<cstdlib>
#include<algorithm>
#include<cmath>
using namespace std;
const double eps=1e-8;

struct segment
{
    double k,b;int id;
}seg[510000],sta[510000];int top;
bool cmp(segment sg1,segment sg2)
{
    return (sg1.k!=sg2.k)?sg1.k<sg2.k:sg1.b>sg2.b;
}
bool cmd(segment sg1,segment sg2)
{
    return sg1.id<sg2.id;
}

double jdx(segment sg1,segment sg2)
{
    return (sg1.b-sg2.b)/(sg2.k-sg1.k);
}
int main()
{
    int n;
    scanf("%d",&n);
    for(int i=1;i<=n;i++)
        scanf("%lf%lf",&seg[i].k,&seg[i].b), seg[i].id=i;
    sort(seg+1,seg+n+1,cmp);
    int tp=1;
    for(int i=2;i<=n;i++)
        if((seg[tp].k-seg[i].k)>eps)seg[++tp]=seg[i];
    
    top=2;
    sta[1]=seg[1];sta[2]=seg[2];
    for(int i=3;i<=n;i++)
    {
        while(top>1&&jdx(sta[top-1],sta[top])>=jdx(sta[top-1],seg[i]))top--;
        sta[++top]=seg[i];
    }
    
    sort(sta+1,sta+top+1,cmd);
    for(int i=1;i<=top;i++)printf("%d ",sta[i].id);
    return 0;
}