【BZOJ】1007: [HNOI2008]Horizontal visible line (convex hull)

topic

Portal: QWQ

 

 

analyze

　　maintain a convex hull underneath

　　Long time no blogging...  

 

 

code

#include <bits/stdc++.h>
using namespace std;
const int maxn=500005;
const double eps=1e-15,INF=1e10;
struct Line{
    double a,b;int n;
}l[maxn];
Line st[maxn];int top=0;
bool cmp(Line a,Line b){
    if(fabs(a.a-b.a)<eps)return a.b<b.b;
    return a.a<b.a;
}
bool cmp2(Line a,Line b){ return a.n<b.n; }
double cross(Line x,Line y){//交点的x值 
    return (y.b-x.b)/(x.a-y.a);
}
intmain ()
{
    int n;
    scanf("%d",&n); 
    for(int i=1;i<=n;i++){
        scanf("%lf%lf",&l[i].a,&l[i].b);
        l[i].n=i;
    }
    sort(l+1,l+1+n,cmp);
    
    for(int i=1;i<=n;i++){
    //    printf("--- %d\n",top);
        while((top>1 && cross(st[top-1],st[top])>=cross(st[top-1],l[i]) ) || (top>0 && fabs(st[top].a-l[i].a)<eps)){
            top--;
        }
        st[++top]=l[i];
    }
    
    sort(st+1,st+1+top,cmp2);
    for(int i=1;i<=top;i++) printf("%d ",st[i].n);
    return 0;
}