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 1000000001 using 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; }