POJ 2187 Beauty Contest (凸包)

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题目地址：POJ 2187

凸包第一发。。用的大白书上的andew算法。

先求出凸包，然后最大距离一定是凸包之中的某两点之间的距离，然后枚举找出最大值。

代码如下：

#include <iostream>#include <cstdio>#include <string>#include <cstring>#include <stdlib.h>#include <math.h>#include <ctype.h>#include <queue>#include <map>#include <set>#include <algorithm>using namespace std;#define LL long longconst int INF=0x3f3f3f3f;int top, n;struct Point{    int x, y;}p[100000], tu[100000];int dist(Point x, Point y){    return (x.x-y.x)*(x.x-y.x)+(x.y-y.y)*(x.y-y.y);}Point operator - (Point x, Point y){    Point z;    z.x=x.x-y.x;    z.y=x.y-y.y;    return z;}int Cross(Point x, Point y){    return x.x*y.y-x.y*y.x;}int cmp(Point x, Point y){    if(x.x==y.x) return x.y<y.y;    return x.x<y.x;}int Max(int x, int y){    return x>y?x:y;}void Andew(){    int i, j, k;    sort(p,p+n,cmp);    top=0;    for(i=0;i<n;i++)    {        while(top>1&&Cross(tu[top-1]-tu[top-2],p[i]-tu[top-1])<=0) top--;        tu[top++]=p[i];    }    k=top;    for(i=n-2;i>=0;i--)    {        while(top>k&&Cross(tu[top-1]-tu[top-2],p[i]-tu[top-1])<=0) top--;        tu[top++]=p[i];    }    int max1=-1;    for(i=0;i<top;i++)    {        for(j=0;j<i;j++)        {            max1=Max(max1,dist(tu[i],tu[j]));        }    }    printf("%d\n",max1);}int main(){    int i;    scanf("%d",&n);    for(i=0;i<n;i++)    {        scanf("%d%d",&p[i].x,&p[i].y);    }    Andew();    return 0;}

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