Sort it(树状数组逆序对)
http://acm.hdu.edu.cn/showproblem.php?pid=2689
#include<bits/stdc++.h> #define low(x) (x&(-x)) using namespace std; const int maxn=1e5+5; typedef long long ll; int a[maxn]; int n; void add(int i,int d){ for(;i>0;i-=low(i)) a[i]+=d; } int ask(int i){ int cnt=0; for(;i<=n;i+=low(i)) cnt+=a[i]; return cnt; } int main(){ //freopen("in","r",stdin); while(scanf("%d",&n)!= EOF) { ll ans = 0; memset(a,0,sizeof(a)); for (int i = 1; i <= n; i++) { int num; scanf("%d", &num); ans += ask(num); add(num, 1); } printf("%lld\n",ans); } return 0; }