【树状数组逆序对】USACO.2011JAN-Above the median

 1 #include<iostream>
 2 #include<cstdio>
 3 #include<cstring>
 4 #include<algorithm>
 5 using namespace std;
 6 const int MAXN=10000;
 7 struct Rec
 8 {
 9        int num,orinum; 
10        bool operator < (const Rec &x) const
11        {
12             return num<x.num;
13        }
14 };
15 int n,x;
16 Rec b[MAXN];
17 int a[MAXN],e[MAXN];
18 int ans=0;
19 
20 int lowbit(int x)
21 {
22     return (x & (-x));
23 }
24 
25 void add(int p,int x)
26 {
27      int i=p;
28      while (i<=(2*n+1))
29      {
30            e[i]+=x;
31            i=i+lowbit(i);
32      }
33 }
34 
35 int sum(int p)
36 {
37     int s=0;
38     int i=p;
39     while (i>0)
40     {
41           s+=e[i];
42           i=i-lowbit(i);
43     }
44     return s;
45 }
46 
47 int main()
48 {
49     /*input*/
50     scanf("%d%d",&n,&x);
51     int num[MAXN];
52 
53     a[0]=0;
54     for (int i=1;i<=n;i++)
55     {
56         a[i]=a[i-1];
57         scanf("%d",&num[i]);
58         if (num[i]>=x) a[i]++;
59         if (num[i]<x) a[i]--;
60     }
61     
62     
63     add(n+1,1);
64     for (int i=1;i<=n;i++)
65     {
66         ans+=sum(a[i]+n+1);
67         add(a[i]+n+1,1);
68     }
69     
70     cout<<ans<<endl;
71     
72     return 0;
73 }