[USACO10NOV]奶牛的图片Cow Photographs 树状数组 递推

Code:

#include<cstdio>
#include<algorithm>
#include<string>
#include<cstring>
using namespace std;
const int maxn=1000000+4;
int n,C[maxn];
struct Data_Structure{
	int lowbit(int t){
		return t&(-t);
	}
	void add(int pos,int delta){
		while(pos<=n)C[pos]+=delta, pos+=lowbit(pos);
	}
	int query(int x){
		int sum=0;
		while(x>0){
			sum+=C[x];
			x-=lowbit(x);
		}
		return sum;
	}
}T;
int A[maxn],position[maxn];
int main(){
	scanf("%d",&n);
	for(int i=1;i<=n;++i) scanf("%d",&A[i]), position[A[i]]=i;
	long long last=0;
 
	for(int i=n;i>=1;--i){
		last+=T.query(A[i] -1);
		T.add(A[i],1);
	}
 
	long long best=last;
 
	for(int i=1;i<=n;++i)
		best=min(best,last-(position[i]-1) + (n-position[i])),last=last-(position[i]-1)+(n-position[i]);
	printf("%lld",best);
	return 0;
}