poj3855Blast the Enemy!（多边形重心）

链接

 #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-;
 const double pi = acos(-1.0);
 const double inf = ~0u>>;
 struct point
 {
     double x,y;
     point(double x=,double y = ):x(x),y(y){}
 }p[N];
 typedef point pointt;
 point operator -(point a,point b)
 {
     return point(a.x-b.x,a.y-b.y);
 }
 double cross(point a,point b)
 {
     return a.x*b.y-a.y*b.x;
 }
 int main()
 {
     int i,j,n;
     int kk = ;
     while(scanf("%d",&n)&&n)
     {
         for(i =  ;  i< n; i++)
         scanf("%lf%lf",&p[i].x,&p[i].y);
         p[n] = p[];
         double sx = ,sy = ,sum = ;
         for(i = ; i < n- ;i++)
         {
             double ts = cross(p[i]-p[],p[i+]-p[])/;
             double x = p[].x+p[i].x+p[i+].x;
             double y = p[].y+p[i].y+p[i+].y;
             sum+=ts;
             sx+=x*ts;
             sy+=y*ts;
         }
         printf("Stage #%d: %.6f %.6f\n",++kk,sx/3.0/sum,sy/3.0/sum);
     }
     return ;
 }