http://poj.org/problem?id=3907

題目大意：求多邊形面積，結果四舍五入。

————————————————————

多邊形面積公式板子題。

#include<cstdio>
#include<queue>
#include<cmath>
#include<cstring>
#include
 
<vector>
#include<algorithm>
using namespace std;
const int K=1e4+5;
typedef double dl;
struct point{//既是向量又是點
    dl x;
    dl y;
}q[K];
int n;
inline point getmag(point a,point b){
    point s;
    s.x=b.x-a.x;s.y=b.y-a.y;
    return s;
}
inline dl multiX(point a,point b){
    
 
return a.x*b.y-b.x*a.y;
}
inline dl area(){
    dl ans=0;
    for(int i=1;i<=n;i++){
    ans+=multiX(getmag(q[0],q[i]),getmag(q[0],q[i%n+1]));
    }
    return fabs(ans)/2;
}
int main(){
    q[0].x=0,q[0].y=0;
    while(scanf("%d",&n)!=EOF&&n){
    for(int i=1;i<=n;i++){
        scanf(
 
"%lf%lf",&q[i].x,&q[i].y);
    }
    printf("%d\n",(int)round(area()));
    }
    return 0;
}

POJ3907：Build Your Home——題解