HDU 1394最小逆シーケンス数

#include <iostream>
using namespace std;
#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);
}

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;
}

int x[maxn];

int main()
{
	int n;
	while(scanf("%d", &n) != EOF)
	{
		build(0, n-1, 1);
		int sum = 0;
		for(int i = 0; i < n; i++)
		{
			scanf("%d", &x[i]);
			sum += query(x[i], n-1, 0, n-1, 1);
			update(x[i], 0, n-1, 1);
		}
		int ret = sum;
		for(int i = 0; i < n; i++)
		{
			sum += n - x[i] - x[i] - 1;
			ret = min(ret, sum);
		}
		printf("%d
", ret);
	}
	return 0;
}