hoj 2275 Number sequence

Number sequence
Gven a number sequence which has N element(s)、please callate the number of different collection for three number Ai、Aj、Ak、which satisfy that Ai＜Aj＞Ak and i＜j＞Ak.i
Input
The first line is an integer N(N<=50000).The second line contains N integer(s):A 1,A 2,…,An(0==Ai<=32768).
Output
The e e e is only one number,which is the number of different collection.
Sample Input
51 2 3 4 1
Sample Output
6
テーマは統計シーケンスの中でAiです.  Ak(i 
ACコードは以下の通りです

#include
#include
#include
#define M 50010
using namespace std;

int c[M],num[M];
int l[M],n;

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

void add(int a,int b)
{
    while (a0)
    {
        ans+=c[a];
        a-=lowbit(a);
    }
    return ans;
}

int main ()
{
    int i,j;
    int a,b;
    while(~scanf("%d",&n))
    {
        memset(c,0,sizeof c);
        memset(num,0,sizeof num);
        for(i=1;i<=n;i++)
        {
            scanf("%d",&num[i]);
            l[i]=sum(num[i]-1);
            add(num[i],1);
        }
        memset(c,0,sizeof c);
        long long ans=0;
        for(i=n;i>=1;i--)
        {
            ans=ans+(long long)sum(num[i]-1)*l[i];
            add(num[i],1);
        }
        printf("%lld
",ans);
    }
    return 0;
}