BZOJ1132: [POI2008]Tro

 1 #include<cstdio>
 2 #include<cstdlib>
 3 #include<cmath>
 4 #include<cstring>
 5 #include<algorithm>
 6 #include<iostream>
 7 #include<vector>
 8 #include<map>
 9 #include<set>
10 #include<queue>
11 #include<string>
12 #define inf 1000000000
13 #define maxn 3000+500
14 #define maxm 500+100
15 #define eps 1e-10
16 #define ll long long
17 #define pa pair<int,int>
18 #define for0(i,n) for(int i=0;i<=(n);i++)
19 #define for1(i,n) for(int i=1;i<=(n);i++)
20 #define for2(i,x,y) for(int i=(x);i<=(y);i++)
21 #define for3(i,x,y) for(int i=(x);i>=(y);i--)
22 using namespace std;
23 inline int read()
24 {
25     int x=0,f=1;char ch=getchar();
26     while(ch<‘0‘||ch>‘9‘){if(ch==‘-‘)f=-1;ch=getchar();}
27     while(ch>=‘0‘&&ch<=‘9‘){x=10*x+ch-‘0‘;ch=getchar();}
28     return x*f;
29 }
30 int n;
31 struct rec{ll x,y;double z;}a[maxn];
32 inline double kk(rec a,rec b)
33 {
34     if(b.x==a.x)
35     {
36         if(b.y<a.y)return -inf;else return inf;
37     }
38     return (double)(b.y-a.y)/(double)(b.x-a.x);
39 }
40 inline bool cmp(rec a,rec b)
41 {
42     return a.z<b.z;
43 }
44 int main()
45 {
46     freopen("input.txt","r",stdin);
47     freopen("output.txt","w",stdout);
48     n=read();
49     for1(i,n)a[i].x=read(),a[i].y=read();
50     ll ans=0;
51     for1(i,n-2)
52     {
53       int k=i;    
54       for2(j,i+1,n)if(a[j].x<a[k].x)k=j;
55       swap(a[k],a[i]);
56       for2(j,i+1,n)a[j].z=kk(a[i],a[j]);
57       sort(a+i+1,a+n+1,cmp);    
58       ll xx=0,yy=0;
59       for2(j,i+1,n)
60         {
61           ans+=(a[j].x-a[i].x)*yy-(a[j].y-a[i].y)*xx;
62           xx+=a[j].x-a[i].x;yy+=a[j].y-a[i].y;
63               //cout<<ans<<‘ ‘<<xx<<‘ ‘<<yy<<‘ ‘<<a[j].x<<‘ ‘<<a[j].y<<‘ ‘<<a[j].z<<endl;
64         }              
65     }    
66     printf("%lld",abs(ans)/2);
67     if(ans&1)puts(".5");else puts(".0");
68     return 0;
69 }