[POI 2008][BZOJ 1132]Tro

 1 #include <cstdio>
 2 #include <algorithm>
 3 const int size=5000;
 4 typedef struct point vector;
 5 typedef long long llint;
 6 
 7 namespace IOspace
 8 {
 9     inline int getint()
10     {
11         register int num=0;
12         register char ch;
13         do ch=getchar(); while (ch<‘0‘ || ch>‘9‘);
14         do num=num*10+ch-‘0‘, ch=getchar(); while (ch>=‘0‘ && ch<=‘9‘);
15         return num;
16     }
17     inline void putint(llint num, char ch=‘\n‘)
18     {
19         char stack[20];
20         register int top=0;
21         if (num==0) stack[top=1]=‘0‘;
22         for ( ;num;num/=10) stack[++top]=num%10+‘0‘;
23         for ( ;top;top--) putchar(stack[top]);
24         if (ch) putchar(ch);
25     }
26 }
27 
28 struct point
29 {
30     llint x, y;
31     inline point() {}
32     inline point(llint _x, llint _y):x(_x), y(_y) {}
33     inline vector & operator += (vector v) {x+=v.x; y+=v.y; return *this;}
34     inline vector & operator -= (vector v) {x-=v.x; y-=v.y; return *this;}
35 };
36 point p[size];
37 inline llint operator * (vector a, vector b) {return a.x*b.y-a.y*b.x;}
38 inline bool operator < (point a, point b) {return a*b>0;}
39 inline void swap(point & a, point & b) {point t=a; a=b; b=t;}
40 
41 int N;
42 inline llint count(int);
43 
44 int main()
45 {
46     llint ans=0;
47 
48     N=IOspace::getint();
49     for (int i=1;i<=N;i++) p[i].x=IOspace::getint(), p[i].y=IOspace::getint();
50 
51     for (int i=N;i>=1;i--) ans+=count(i);
52 
53     bool b=ans&1LL;
54     IOspace::putint(ans>>1LL, ‘.‘);
55     IOspace::putint(b?5:0);
56 
57     return 0;
58 }
59 inline llint count(int n)
60 {
61     llint ret=0;
62 
63     int k=1;
64     for (int i=1;i<=n;i++)
65         if (p[i].x<p[k].x || p[i].x==p[k].x && p[i].y<p[k].y)
66             k=i;
67     swap(p[k], p[n]);
68 
69     for (int i=1;i<=n;i++) p[i]-=p[n];
70 
71     std::sort(p+1, p+n);
72 
73     vector s(0, 0);
74     for (int i=1;i<=n;i++)
75     {
76         ret+=s*p[i];
77         s+=p[i];
78     }
79 
80     return ret;
81 }

 1 #include<cstdio>
 2 #include<iostream>
 3 #include<algorithm>
 4 using namespace std;
 5 
 6 #define maxn 30010
 7 typedef long long ll;
 8 struct xllend3
 9 {
10     int x,y;
11 } orz[maxn];
12 
13 int n,m;
14 ll ans;
15 
16 bool cmp(const xllend3 &a,const xllend3 &b)
17 {
18     return a.x*b.y>a.y*b.x;
19 }
20 ll orzhzw(int n)
21 {
22     int i=0;
23     for (int j=0;j<n;j++)
24     if (orz[j].x<orz[i].x||(orz[j].x==orz[i].x && orz[j].y<orz[i].y))
25         i=j;
26     swap(orz[i],orz[n-1]);
27     for (int i=0;i<n-1;i++)
28         orz[i].x-=orz[n-1].x,orz[i].y-=orz[n-1].y;
29     sort(orz,orz+n-1,cmp);
30     ll gui=0;
31     ll sx=0,sy=0;
32     for (int i=n-2;i>=0;i--)
33     {
34         gui+=(ll)orz[i].x*sy-(ll)orz[i].y*sx;
35         sx+=orz[i].x;
36         sy+=orz[i].y;
37     }
38     return gui;
39 }
40     
41 int main()
42 {
43     scanf("%d",&n);
44     for (int i=0;i<n;i++)    scanf("%d%d",&orz[i].x,&orz[i].y);
45     for (int i=n;i>2;i--)    ans+=orzhzw(i);
46     cout<<ans/2;
47     if (ans&1)    cout<<".5"<<endl;else cout<<".0"<<endl;
48     return 0;
49 }