UESTC -- 841 休生伤杜景死惊开（树状数组 逆序数）

#include<cstdio>
#include<cstring>
#include<iostream>
#define Max(a,b) ((a)>(b)?(a):(b))
using namespace std;
const int N=50010;
int n,sum
,a
,b
,c
,maxn;
int lowbit(int x){
return x&(-x);
}
void update(int i,int val){
while(i<=maxn){
sum[i]+=val;
i+=lowbit(i);
}
}
int getsum(int i){
int s=0;
while(i>0){
s+=sum[i];
i-=lowbit(i);
}
return s;
}
int main(){
while(~scanf("%d",&n)){
long long ans=0;
maxn=0;
memset(sum,0,sizeof(sum));
memset(b,0,sizeof(b));
memset(c,0,sizeof(c));
for(int i=1;i<=n;++i) scanf("%d",&a[i]),maxn=Max(maxn,a[i]);
for(int i=1;i<=n;++i){
update(a[i],1);
b[i]=getsum(a[i]-1);
}
memset(sum,0,sizeof(sum));
for(int i=n;i>=1;--i){
update(a[i],1);
c[i]=getsum(a[i]-1);
}
for(int i=1;i<=n;++i) ans+=(b[i]*c[i]);
printf("%lld\n",ans);
}
}