逆序对模板（树状数组）

//P1908 逆序对
#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
const int mxn=5e5+5;
int a[mxn],c[mxn],b[mxn],f[mxn];
int n;
inline int lowbit(int x){
    return x&-x;
}
inline ll get(int x){
    ll sum=0;
    for(;x;x-=lowbit(x)){
        sum+=c[x];
    }
    return sum;
}
inline void motify(int x,int y){
    for(;x<=n;x+=lowbit(x)){
        c[x]+=y;
    }
}
int main(){
    cin>>n;
    for(int i=1;i<=n;i++){
        scanf("%d",&a[i]);
        b[i]=a[i]; 
    }
    sort(b+1,b+n+1);
    int N=unique(b+1,b+n+1)-b-1;
    for(int i=1;i<=n;i++){
        a[i]=lower_bound(b+1,b+N+1,a[i])-b; 
    }
    for(int i=1;i<=n;i++){
        motify(a[i],1);
        f[i]=i-get(a[i]);
    }
    ll ans=0;
    for(int i=1;i<=n;i++){
        ans+=f[i];
    }
    cout<<ans;
    return 0;
}

 

来源：https://www.cnblogs.com/duojiaming/p/11760790.html