http://acm.hdu.edu.cn/showproblem.php?pid=1394&&线段树之求逆序数

#include<iostream>
#include<string.h>
#include<cstdio>
#include<algorithm>
#define N 5005
#define L(x)(x<<1)
#define R(x) (x<<1|1)
#define M(x,y) ((x+y)>>1)
using namespace std;
typedef struct str
{
	int l;
	int r;
	int key;
}Node;
Node node[N<<1];
int value[N];
void bulid(int t,int l,int r)
{
	node[t].l=l;
	node[t].r=r;
	node[t].key=0;
	if(l==r-1)return ;
	int mid=M(l,r);
	bulid(L(t),l,mid);
	bulid(R(t),mid,r);
}
void update(int t,int a)
{
	node[t].key++;
	if(node[t].l==(node[t].r-1))  return;
	int mid=M(node[t].l,node[t].r);
	if(a>=mid) update(R(t),a);
	else update(L(t),a);
}
int Quary(int t,int a,int b)
{
	if(node[t].l==a&&node[t].r==b) return node[t].key;
	int mid=M(node[t].l,node[t].r);
	if(a>=mid) return Quary(R(t),a,b);
	else
	{
		if(b<=mid) return Quary(L(t),a,b);
		else return  Quary(L(t),a,mid)+Quary(R(t),mid,b);
	}
}
int main()
{
	int n;
	while(~scanf("%d",&n))
	{ 
		int sum=0;
		bulid(1,0,n);
		
		for(int i=1;i<=n;++i)
		{  
			scanf("%d",&value[i]);
			sum+=Quary(1,value[i],n);
		     update(1,value[i]);
		}
		int k=sum;
		for(int i=n;i>=2;--i)
		{
			 sum=sum+value[i]-(n-1-value[i]);
			if(sum<k) k=sum;
		}
		printf("%d\n",k);
	}return 0;
}