Asia Yokohama Regional Contest 2018 G题 What Goes Up Must Come Down（树状数组求逆序对）

#define bug(x,y) cout<<"i="<<x<<": "<<y<<endl
#define IO std::ios::sync_with_stdio(0);
#include <bits/stdc++.h>
#define itor ::iterator
using namespace  std;
typedef long long ll;
typedef pair<ll,ll>P;
#define pb push_back
#define se second
#define fi first
#define rs o*2+1
#define ls o*2
const int N=1e5+5;
int n;
int c[N];
int low(int x){
    return x&(-x);
}
void add(int x,int y){
    while(x<N){   
        c[x]+=y;
        x+=low(x);
    }
}
int cal(int x){
    int res=0;
    while(x){
        res+=c[x];
        x-=low(x);
    }
    return res;
}
vector<int>v[N];
int main(){
    scanf("%d",&n);
    ll ans=0;
    for(int i=1;i<=n;i++){
        int x;
        scanf("%d",&x);
        v[x].pb(i);
        add(i,1);
    }
    for(int i=1;i<N;i++){
        if(v[i].empty())continue;
        for(int j=0;j<v[i].size();j++){
            int x=v[i][j];
            add(x,-1);
            n--;
        }
        for(int j=0;j<v[i].size();j++){  
            int x=v[i][j];
            ll res=min(cal(x-1),n-cal(x));
            //bug(i,res);
            ans+=res;
        }
    }
    printf("%lld\n",ans);
}
/*
7
3 1 4 1 5 9 2
*/