树状树组离散化求逆序对模板

Posted 2022-08-08 hh13579

#include <iostream>
#include <cstring>
#include <stack>
#include <cstdio>
#include <cmath>
#include <queue>
#include <algorithm>
#include <vector>
#include <set>
#include <map>
using namespace std;
#define ll long long
const int  N=500005;
int a[N],b[N],c[N],d[N],n,m=0;
void discrete()

    for(int i=1;i<=n;i++)
        d[i]=a[i];
    sort(d+1,d+n+1);
    for(int i=1;i<=n;i++)
    
        if(i==1||d[i]!=d[i-1])
            b[++m]=d[i];    
        

int query(int x)

    return lower_bound(b+1,b+m+1,x)-b;

int ask(int x)

    int ans=0;
    for(;x;x-=x&-x)
    
        ans=ans+c[x];
    
    return ans;

void add(int x,int y)

    for(;x<=N-1;x+=x&-x)
        c[x]+=y; 

int main()

    ll ans=0;
    cin>>n;
      for(int i=1;i<=n;i++)
      
          scanf("%d",&a[i]);
    
    discrete();
    for(int i=1;i<=n;i++)
    
        add(query(a[i]),1);
        ans+=i-ask(query(a[i]));
    
    cout<<ans<<"\n";
    return 0;