hdu 2491 pingpong

樹狀數組。

#include<iostream>#include<cstdio>#include<cstring>using namespace std;#define maxn 100006int a[maxn];int n=maxn;int lowbit(int i){return i&(-i);}int Sum(int i){int sum=0;while(i>=1){sum+=a[i];i-=lowbit(i);}return sum;}void Update(int i,int tem){while(i<=n){a[i]+=tem;i+=lowbit(i);}}int main(){__int64 sum;int t;int m,i;int b[20004],zx[20004],zd[20004],yx[20004],yd[20004];scanf("%d",&t);while(t--){scanf("%d",&m);for(i=1;i<=m;i++)scanf("%d",&b[i]);memset(a,0,sizeof(a));for(i=1;i<=m;i++){Update(b[i],1);zx[i]=Sum(b[i]-1);zd[i]=Sum(maxn-2)-Sum(b[i]);}memset(a,0,sizeof(a));for(i=m;i>=1;i--){Update(b[i],1);yx[i]=Sum(b[i]-1);yd[i]=Sum(maxn-2)-Sum(b[i]);}sum=0;for(i=1;i<=m;i++)sum+=zx[i]*yd[i]+zd[i]*yx[i];printf("%I64d\n",sum);}return 0;}