线段树-HDU1394Minimum Inversion Number

题目大意和暴力法的数学说明在这http://blog.youkuaiyun.com/sinianluoye/article/details/23766697

这里给出线段树的优化代码  用时比暴力少了一半多

线段树代码摘自http://www.notonlysuccess.com/index.php/segment-tree-complete/

#include<iostream>
#include<cstdio>
#include<cstring>
#include<cmath>
#include <algorithm>
#define ll long long
#define eps 1e-8
#define ms(x,y) (memset(x,y,sizeof(x)))
#define fr(i,x,y) for(int i=x;i<=y;i++)
using namespace std;
/*线段树 点修改 区间求和*/
#define lson l , m , rt << 1
#define rson m + 1 , r , rt << 1 | 1
const int maxn = 5555;
int sum[maxn<<2];
void PushUP(int rt) {
	sum[rt] = sum[rt<<1] + sum[rt<<1|1];
}
void build(int l,int r,int rt) {
	sum[rt]=0;
	if(l==r)return;
	int m = (l + r) >> 1;
	build(lson);
	build(rson);
	PushUP(rt);
}
void update(int p,int l,int r,int rt) {
	if (l == r) {
		sum[rt] ++;
		return ;
	}
	int m = (l + r) >> 1;
	if (p <= m) update(p , lson);
	else update(p , rson);
	PushUP(rt);
}
int query(int L,int R,int l,int r,int rt) {
	if (L<=l&&r<=R) {
		return sum[rt];
	}
	int m=(l+r)>>1;
	int ret = 0;
	if (L<=m) ret+=query(L,R,lson);
	if (R>m) ret+=query(L,R,rson);
	return ret;
}
/*end-线段树 点修改 区间求和*/
int n,a[maxn];
void init()
{
    build(0,n-1,1);
}
void doit()
{
    int t=0;
    fr(i,0,n-1)
    {
        scanf("%d",&a[i]);
        t+=query(a[i],n-1,0,n-1,1);
        update(a[i],0,n-1,1);
    }
    int ans=t;
    fr(i,0,n-1)
    {
        t+=n-a[i]*2-1;
        ans=min(t,ans);
    }
    cout<<ans<<endl;
}
int main()
{
    //freopen("in.txt","r",stdin);
    while(cin>>n)
    {
        init();
        doit();
    }
    return 0;
}