hoj 2275 Number Sequence

#include <iostream>
#include <cstring>
#include <cstdio>
#define MAXN 50010

using namespace std;

int N;
int c[MAXN], a[MAXN], b[MAXN], d[MAXN];

int lowbit( int x )
{
return x&(-x);
}

void UFset(int i, int data)
{
while(i <= 32768)
{
c[i] += data;
i += lowbit(i);
}
}

int Querry( int x )
{
int sum = 0;
while(x > 0)
{
sum += c[x];
x -= lowbit(x);
}
return sum;
}

int main()
{
int i;
long long ans;
while(scanf("%d", &N) != EOF)
{
memset(a, 0, sizeof(a));
memset(b, 0, sizeof(b));

memset(c, 0, sizeof(c));
for(i = 0; i < N; ++i)
{
scanf("%d", &d[i]);
a[i] += Querry( d[i] );//在输入的时候就计算i之前的值比它小的值有多少个
UFset( d[i] + 1, 1 );//然后再更新
}

memset(c, 0, sizeof(c));
for(i = N-1; i >= 0; --i)
{
b[i] = Querry(d[i]);
UFset(d[i] + 1, 1);
}
ans = 0;
for(i = 0; i < N; ++i)
ans += (long long)a[i] * (long long)b[i];
printf("%lld\n", ans);
}
return 0;
}