poj 2187 凸包+平面上点之间最大距离

/*  这道题目是算一个平面内的一些点的之间最大的距离，暴力肯定会超时，最远距离，一定是凸包上的两个点的距离，  先用模板找到凸包线上的点，然后枚举任意两点之间的距离，求出最大的距离！*/#include <iostream>#include <cstdio>#include <cstring>#include <cmath>#include <algorithm>using namespace std;const int maxn=50005;int n;double len[maxn];struct point{    double x;    double y;    double there;} p[maxn],chs[maxn];bool cmp1(point a,point b){    if(a.y==b.y)return a.x<b.x;    return a.y<b.y;}double get_there(point a,point b){    return atan2((a.y-b.y),(a.x-b.x));}bool cmp2(point a,point b){    if(a.there==b.there)        return a.x<b.x;    return a.there<b.there;}double dist_point2(point a,point b){    return (b.x-a.x)*(b.x-a.x)+(b.y-a.y)*(b.y-a.y);}bool line_point1(point a,point b,point c){    double m;    m=(b.x-a.x)*(c.y-a.y)-(b.y-a.y)*(c.x-a.x);    if(m>0)return false;    if(m<0)return true;    if(m==0)return true;    return false;}int main(){    while(scanf("%d",&n)!=EOF)    {        double  len;        double max=0;        int i,j;        for(i=0; i<n; i++)        {            scanf("%lf %lf",&p[i].x,&p[i].y);        }        sort(p,p+n,cmp1);//排序，找基点        for(i=0; i<n; i++)        {            p[i].there=get_there(p[i],p[0]);//算极角        }        sort(p+1,p+n,cmp2);//极角排序        chs[0]=p[0];//GrahamScan算法        chs[1]=p[1];        chs[2]=p[2];        int top=2;        for(i=3; i<n; i++)        {            while(top>=1&&line_point1(chs[top-1],chs[top],p[i]))            {                top--;            }            chs[++top]=p[i];        }        top++;        for(j=0; j<top; j++)//就是这里枚举最大的距离        {            for(i=j+1; i<top; i++)            {                len=dist_point2(chs[j],chs[i]);                if(len>max)                    max=len;            }        }        printf("%.0lf\n",max);    }    return 0;}