51NOD——N 1107 斜率小于0的连线数量

 1 #include <algorithm>
 2 #include <cstdio>
 3 
 4 using namespace std;
 5 
 6 const int N(50000+5);
 7 int n,maxn=1e9;
 8 struct Node
 9 {
10     int x,y,mark;
11 }node[N],use[N];
12 bool cmp1(Node a,Node b)
13 {
14     if(a.x==b.x) return a.y<b.y;
15     return a.x<b.x;
16 }
17 bool cmp2(Node a,Node b)
18 {
19     if(a.y==b.y) return a.mark<b.mark;
20     return a.y<b.y;
21 }
22 
23 #define LL long long
24 #define lowbit(x) (x&((~x)+1))
25 LL ans,t[N];
26 void up(int x)
27 {
28     for(;x<=N+1;x+=lowbit(x)) t[x]++;
29 }
30 LL query(int x)
31 {
32     LL ret=0;
33     for(;x;x-=lowbit(x)) ret+=t[x];
34     return ret;
35 }
36 
37 int main()
38 {
39     scanf("%d",&n);
40     for(int i=1;i<=n;i++)
41     {
42         scanf("%d%d",&node[i].x,&node[i].y);
43         node[i].x++; node[i].y++;;
44     }
45     sort(node+1,node+n+1,cmp1);
46     for(int i=1;i<=n;i++) use[i].y=node[i].y,use[i].mark=i;
47     sort(use+1,use+n+1,cmp2);
48     for(int i=n;i>0;up(use[i--].mark))
49         ans+=query(use[i].mark);
50     printf("%lld\n",ans);
51     return 0;
52 }