http://acm.hdu.edu.cn/showproblem.php?pid=2689

树状数组求逆序数应用，，不解释。。

#include<iostream>#include<cstdio>#include<string.h>#include<algorithm>#define M 1000000001using namespace std;typedef long long L;int  s[M];int lowbit(int x){ return x&(-x);}void update(int x){  while(x<=M)   {  s[x]++;      x+=lowbit(x);   }}int Quary(int x){   int sum=0;while(x>0)   {  sum+=s[x];   x-=lowbit(x);   }return sum;}int main(){ int n,T;   scanf("%d",&T);   while(T--)   {   scanf("%d",&n);   memset(s,0,sizeof(s));   int a;      L sum=0;      for(int i=1;i<=n;++i)  {  scanf("%d",&a);      update(a);  sum+=(i-Quary(a));      }  printf("%d\n",sum);   }return 0;}