#include<cstdio>#include<cstring>#include<algorithm>#include<iostream>using namespace std;#define M 35000#define N 50050#define ll long long#define lowbit(x) (x&-x)int tree[M];int num[N];ll tmp[N];int sum(int rt){int s=0;while(rt>0){s+=tree[rt];rt-=lowbit(rt);}return s;}void update(int rt){while(rt<=M-1){tree[rt]++;rt+=lowbit(rt);}}int main(){int n;while(~scanf("%d",&n)){memset(tree,0,sizeof(tree));for(int i=1;i<=n;i++){scanf("%d",&num[i]);num[i]++;}ll ans=0;for(int i=1;i<=n;i++){tmp[i]= sum(num[i]-1);update(num[i]);}memset(tree,0,sizeof(tree));for(int i=n;i>=1;i–){ans+=tmp[i]*(ll)sum(num[i]-1);update(num[i]);}printf("%lld\n",ans);}return 0;}
，美不美乡中水，亲不亲故乡人。
