poj1859The Perfect Symmetry

链接

按x或y排序，假如有对称点的话，头尾相对。

#include <iostream>
#include<cstdio>
#include<cstring>
#include<algorithm>
#include<stdlib.h>
#include<vector>
#include<cmath>
#include<queue>
#include<set>
using namespace std;
#define N 100000
#define LL long long
#define INF 0xfffffff
const double eps = 1e-8;
const double pi = acos(-1.0);
const double inf = ~0u>>2;
struct Point
{
    double x,y;
    Point(double x=0,double y=0):x(x),y(y) {}
}p[N];
typedef Point pointt;
pointt operator + (Point a,Point b)
{
    return Point(a.x+b.x,a.y+b.y);
}
pointt operator - (Point a,Point b)
{
    return Point(a.x-b.x,a.y-b.y);
}
int dcmp(double x)
{
    if(fabs(x)<eps) return 0;
    else return x<0?-1:1;
}
bool cmp(Point a,Point b)
{
    if(dcmp(a.x-b.x)==0)
    return a.y<b.y;
    return a.x<b.x;
}
int main()
{
    int n,i,j;
    while(scanf("%d",&n)&&n)
    {
        for(i = 1; i <= n ;i++)
        scanf("%lf%lf",&p[i].x,&p[i].y);
        sort(p+1,p+n+1,cmp);
        double tx = (p[1].x+p[n].x)/2,ty = (p[1].y+p[n].y)/2;
        int flag =1 ;
        for(i = 2 ; i <= n; i++)
        {
            double x = (p[i].x+p[n-i+1].x)/2;
            double y = (p[i].y+p[n-i+1].y)/2;
            if(dcmp(x-tx)!=0||dcmp(y-ty)!=0)
            {
                flag = 0;
                break;
            }
        }
        if(flag)
        printf("V.I.P. should stay at (%.1f,%.1f).
",tx,ty);
        else
        printf("This is a dangerous situation!
");
    }
    return 0;
}