大于它的个数和小于它的个数（树状数组）

#include <bits/stdc++.h>
#define ll long long
using namespace std;
int n,m;
const int N=100005;
ll cnt[N],arr[N],brr[N];
vector<ll> vec;
int getid(ll x) { return lower_bound(vec.begin(),vec.end(),x)-vec.begin()+1; }
int lowbits(int x) { return x&-x; }
void add(int x,int value){ while(x<=N) cnt[x]+=value,x+=lowbits(x); }
int query(int x){
    int sum=0;
    while(x>0){ sum+=cnt[x],x-=lowbits(x); }
    return sum;
}
int main(){
    scanf("%d",&n);
    for(int i=1;i<=n;i++) scanf("%lld",&arr[i]),vec.push_back(arr[i]);
    sort(vec.begin(),vec.end());vec.erase(unique(vec.begin(),vec.end()),vec.end());
    ll sum=0;
    for(int i=1;i<=n;i++){
        add(getid(arr[i]),1);
        brr[i]+=i-query(getid(arr[i]));  //i-小于等于的它的个数就是前面大于它的个数
    }
//    memset(brr,0,sizeof(brr));
    memset(cnt,0,sizeof(cnt));
    for(int i=n;i>=1;i--){
        add(getid(arr[i]),1);
        brr[i]+=query(getid(arr[i])-1);   //后面小于它的个数
//        cout << brr[i] << endl;
    }
    for(int i=1;i<=n;i++){
        sum+=1LL*(1+brr[i])*brr[i]/2;
    }
    cout << sum << endl;
    return 0;
}