HDU 1394 Minimum Inversion Number(单点更新)

转载请注明出处：忆梦http://blog.csdn.net/yimeng2013/article/details/17190083

#include<cstdio>
#include<algorithm>
using namespace std;
#define lson l , m, rt << 1
#define rson m+1, r, rt<<1|1
#define maxn 5555
int sum[maxn<<2];
int num[maxn];
void PushUp(int rt)
{
sum[rt] = sum[rt<<1] + sum[rt<<1|1];
}

void build(int l, int r, int rt)
{
if(l == r)
{
sum[rt]= 0;
return ;
}

int m = (l + r) >> 1;
build(lson);
build(rson);
PushUp(rt);
}

int query(int L, int R, int l, int r, int rt)
{
if(L <= l && r <= R)
{
return sum[rt];
}
int ret = 0;
int m = (l + r) >> 1;
if(L <= m)
ret += query(L, R, lson);
if(R > m)
ret += query(L, R, rson);
return ret;
}

void update(int pos, int l, int r, int rt)
{
if(l == r)
{
sum[rt]++;
return;
}
int m = (l + r) >> 1;
if(pos <= m)
update(pos,  lson);
else
update(pos,  rson);
PushUp(rt);
}
int main ()
{
int n;
while(scanf("%d", &n)!= EOF)
{
for(int i = 1; i <= n; i++)
{
scanf("%d", &num[i]);
num[i]++;
}

build(1, n, 1);

int ret = 0;
for(int i = 1; i <= n; i++)
{
ret += query(num[i], n, 1, n, 1);
update(num[i], 1, n, 1);
}

int ans = ret;
for(int i = 1; i < n; i++)
{
ret += n - 2*num[i] + 1;
ans = min(ans, ret);
}
printf("%d\n", ans);
}
return 0;
}