叉乘的性質

 1 #include<cstdio>
 2 #include<cstring>
 3 #include<iostream>
 4 #include<algorithm>
 5 using namespace std;
 6 
 7 int n,x[406],y[406];
 8 int ans;
 9 
10 int main(){
11     freopen("areas.in","r",stdin);
12     freopen("areas.out","w",stdout);
13     scanf("%d",&n);
 
14     for(int i=1;i<=n;++i)
15         scanf("%d%d",&x[i],&y[i]);
16     for(int i=2;i<=n;++i)
17         ans+=(x[i-1]*y[i]-x[i]*y[i-1]);
18     ans+=(x[n]*y[1]-x[1]*y[n]);
19     printf("%d",ans/2);
20     //while(1);
21     return 0;
22 }

cogs 1229. 多邊形面積