逆序对

这也能用归并做，不过有另一道题，这里就先用树状数组

#include<iostream>
#include<algorithm>
using namespace std;
const int N=5e5+5;
typedef long long ll;
struct num{
int x,id;
}a[N];
ll c[N],n,m,ans;
int lowbit(int x){
    return x & (-x);
}
void update(int x,int v){
    while(x<=n){
        c[x]+=v;
        x+=lowbit(x);
    }
}
ll sum(int x){
    ll ans=0;
    while(x>0){
        ans+=c[x];
        x-=lowbit(x);
    }
    return ans;
}
bool cmp(num a,num b){
    if(a.x!=b.x)return a.x>b.x;
    return a.id>b.id;
}
int main(){
    cin>>n;
    for(int i=1;i<=n;i++){
        cin>>a[i].x;
        a[i].id=i;
    }
    sort(a+1,a+n+1,cmp);
    for(int i=1;i<=n;i++){
        update(a[i].id,1);
        ans+=sum(a[i].id-1);//防止记录到自己，所以-1
    }
    cout<<ans;
    return 0;
}