树状数组求逆序对模板

Posted 2022-09-09 背着代码的蜗牛

#include<iostream>
#include<algorithm>
using namespace std;
const int maxn=40010;
int n,ans,a[maxn],b[maxn],c[maxn],s[maxn];
int lowbit(int x)

    return x&(-x);

void add(int x)

    for(int i=x;i<=n;i+=lowbit(i))
    c[i]++;

int getsum(int x)

    int ans=0;
    for(int i=x;i>=1;i-=lowbit(i))
    ans+=c[i];
    return ans;

int main()

    cin>>n;
    for(int i=1;i<=n;i++)
    cin>>a[i],s[i]=a[i];
    sort(s+1,s+n+1);
    int tot=unique(s+1,s+n+1)-s-1;
    for(int i=1;i<=n;i++)
    
        int pos=lower_bound(s+1,s+tot+1,a[i])-s;
        b[i]=pos;
    
    for(int i=n;i>=1;i--)
    
        ans+=getsum(b[i]);
        add(b[i]+1);
    
    cout<<ans;
    return 0;