CodeForces 598C Nearest vectors

这题对精度要求很高。用atan2吧。。。

#include<iostream>
#include<cstring>
#include<cmath>
#include<algorithm>
#include<cstdio>
using namespace std;

const int maxn=100000+10;
int n;
struct X
{
    long double x,y;
    long double k;
    int id;
}s[maxn];

bool cmp(const X&a,const X&b)
{
    return a.k<b.k;
}

int main()
{
    scanf("%d",&n);
    for(int i=1;i<=n;i++)
    {
        cin>>s[i].x>>s[i].y;
        s[i].k=atan2(s[i].y,s[i].x);
        s[i].id=i;
    }

    sort(s+1,s+1+n,cmp);

    s[0]=s[n];



    long double Min=99999;
    int ans1,ans2;
    for(int i=0;i<n;i++)
    {
        long double jiao=s[i+1].k-s[i].k;
        if(jiao<0) jiao+=acos(-1.0)*2;
        if(jiao<Min)
        {
            Min=jiao;
            ans1=s[i].id;
            ans2=s[i+1].id;
        }
    }
    printf("%d %d\n",ans1,ans2);
    return 0;
}