hdu 5147 Sequence II

http://acm.hdu.edu.cn/showproblem.php?pid=5147

 题意：问有多少个这样的四元组(a,b,c,d),满足条件是 1<=a<b<c<d; Aa<Ab; Ac<Ad;

思路：用树状数组求，从右向左求在这个数之前形成多少个逆序数对记录在r数组里面，然后在从左向右求出在输入这个数之前形成多少个逆序数对存在l数组里面，然后枚举b就行；

#include <cstdio>
#include <cstring>
#include <algorithm>
#define maxn 50001
#define ll long long
using namespace std;

int t;
int n;
int a[maxn];
int c[maxn];
ll l[maxn],r[maxn],sum[maxn];

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

void insert(int x,int v)
{
    while(x<maxn)
    {
        c[x]+=v;
        x+=lowbit(x);
    }
}

ll getsum(int x)
{
    ll ans=0;
    while(x>0)
    {
        ans+=c[x];
        x-=lowbit(x);
    }
    return ans;
}

int main()
{
    scanf("%d",&t);
    while(t--)
    {
        memset(sum,0,sizeof(sum));
        memset(a,0,sizeof(a));
        scanf("%d",&n);
        for(int i=1; i<=n; i++)
        {
            scanf("%d",&a[i]);
        }
        memset(c,0,sizeof(c));
        sum[n+1]=0;
        for(int i=n; i>=1; i--)
        {
            r[i]=getsum(n)-getsum(a[i]);
            insert(a[i],1);
            sum[i]=sum[i+1]+r[i];
        }
        memset(c,0,sizeof(c));
        for(int i=1; i<=n; i++)
        {
            l[i]=getsum(a[i]-1);
            insert(a[i],1);
        }
        ll ans=0;
        for(int i=2; i<=n; i++)
        {
            ans+=(l[i]*sum[i+1]);
        }
        printf("%lld
",ans);
    }
    return 0;
}