题解

求出逆序数,O(N)遍历一遍
#include<iostream>
#include<cstdio>
#include<cstdlib>
#include<cmath>
#include<cstring>
#include<algorithm>
#define rep(i,x,y) for(register int i=x;i<=y;++i)
#define repd(i,x,y) for(register int i=x;i>=y;--i)
#define lowbit(x) x&(-x)
using namespace std;
const int N=1e5;
const int inf=0x3f3f3f3f;
typedef pair<int,int> s;
int tr[N],a[N],n,res,ans;
s q[N];
inline void update(int x,int y){for(register int i=x;i<=n;i+=lowbit(i))tr[i]+=y;}
inline int query(int x){int ans=0;for(register int i=x;i;i-=lowbit(i))ans+=tr[i];return ans;}
int main(){
	while(~scanf("%d",&n)){
		ans=inf;res=0;
		memset(tr,0,sizeof(tr));
		rep(i,1,n){scanf("%d",&a[i]);q[i]=s(a[i],i);}
		sort(q+1,q+n+1);
		rep(i,1,n){update(q[i].second,1);res+=i-query(q[i].second);}
		if(ans>res)ans=res;
		rep(i,1,n){res=res-a[i]+n-a[i]-1;ans=min(ans,res);}
		printf("%d\n",ans);
	}
	return 0;
}