对点排序，按顺序往后找，找之前极角排序，保证面积为正
1 #include<algorithm> 2 #include<iostream> 3 #include<cstdlib> 4 #include<cstring> 5 #include<cstdio> 6 #include<string> 7 #include<cmath> 8 #include<ctime> 9 #include<queue> 10 #include<stack> 11 #include<map> 12 #include<set> 13 #define rre(i,r,l) for(int i=(r);i>=(l);i--) 14 #define re(i,l,r) for(int i=(l);i<=(r);i++) 15 #define Clear(a,b) memset(a,b,sizeof(a)) 16 #define inout(x) printf("%d",(x)) 17 #define douin(x) scanf("%lf",&x) 18 #define strin(x) scanf("%s",(x)) 19 #define LLin(x) scanf("%lld",&x) 20 #define op operator 21 #define CSC main 22 typedef unsigned long long ULL; 23 typedef const int cint; 24 typedef long long LL; 25 using namespace std; 26 void inin(int &ret) 27 { 28 ret=0;int f=0;char ch=getchar(); 29 while(ch<'0'||ch>'9'){if(ch=='-')f=1;ch=getchar();} 30 while(ch>='0'&&ch<='9')ret*=10,ret+=ch-'0',ch=getchar(); 31 ret=f?-ret:ret; 32 } 33 struct xl 34 { 35 LL x,y; 36 xl(LL x=0,LL y=0):x(x),y(y){} 37 void in(){LLin(x),LLin(y);} 38 bool op < (const xl &a)const {return x==a.x?y<a.y:x<a.x;} 39 xl op + (const xl &a)const {return xl(x+a.x,y+a.y);} 40 xl op - (const xl &a)const {return xl(x-a.x,y-a.y);} 41 xl op * (const LL &a)const {return xl(x*a,y*a);} 42 xl op / (const LL &a)const {return xl(x/a,y/a);} 43 }; 44 LL D_(const xl &a,const xl &b){return a.x*b.x+a.y*b.y;} 45 LL X_(const xl &a,const xl &b){return a.x*b.y-a.y*b.x;} 46 bool com(const xl &a,const xl &b) 47 { 48 return X_(a,b)>0; 49 } 50 xl p[3030]; 51 xl sta[3030];int top; 52 int n; 53 int main() 54 { 55 inin(n); 56 re(i,1,n)p[i].in(); 57 LL ans=0; 58 sort(p+1,p+n+1); 59 re(i,1,n-2) 60 { 61 top=0;xl temp=xl(0,0); 62 re(j,i+1,n)sta[++top]=p[j]-p[i]; 63 sort(sta+1,sta+top+1,com); 64 re(j,1,top) 65 { 66 ans+=X_(temp,sta[j]); 67 temp=temp+sta[j]; 68 } 69 } 70 printf("%lld.%d",ans>>1,ans&1?5:0); 71 return 0; 72 }