树状数组:

维护每一个数前面比它小的数的个数,和这个数后面比他大的数的个数

再枚举每个位置组合一下 

Sequence II

Time Limit: 5000/2500 MS (Java/Others)    Memory Limit: 32768/32768 K (Java/Others)
Total Submission(s): 121    Accepted Submission(s): 58

1 5 1 3 2 4 5

4

/* *********************************************** Author        :CKboss Created Time  :2014年12月20日 星期六 21时38分00秒 File Name     :HDOJ5147.cpp ************************************************ */  #include <iostream> #include <cstdio> #include <cstring> #include <algorithm> #include <string> #include <cmath> #include <cstdlib> #include <vector> #include <queue> #include <set> #include <map>  using namespace std;  typedef long long int LL;  const int maxn=55000;  int a[maxn]; int n;  LL sum1[maxn],sum2[maxn]; int t1[maxn],t2[maxn];  int lowbit(int x) { return x&(-x); }  /// 1 找比当前数小的 2 找比当前数大的  void init() {  memset(sum1,0,sizeof(sum1));  memset(sum2,0,sizeof(sum2));  memset(t1,0,sizeof(t1));  memset(t2,0,sizeof(t2)); }  void add(int kind,int p) {  if(kind==1) for(int i=p;i<maxn;i+=lowbit(i)) t1[i]+=1;  else if(kind==2) for(int i=p;i;i-=lowbit(i)) t2[i]+=1; }  int sum(int kind,int p) {  int ret=0;  if(kind==1) for(int i=p;i;i-=lowbit(i)) ret+=t1[i];  else if(kind==2) for(int i=p;i<maxn;i+=lowbit(i)) ret+=t2[i];  return ret; }  int main() {     //freopen("in.txt","r",stdin);     //freopen("out.txt","w",stdout);   int T_T;  scanf("%d",&T_T);  while(T_T--)  {   scanf("%d",&n);   for(int i=1;i<=n;i++) scanf("%d",a+i);    init();    /// from left to right   for(int i=1;i<=n;i++)   {    int ss=sum(1,a[i]);    sum1[i]=ss;    add(1,a[i]);   }   /// from right to left   for(int i=n;i>=1;i--)   {    int ss=sum(2,a[i]);    sum2[i]=sum2[i+1]+ss;    add(2,a[i]);   }    LL ans=0;   for(int i=2;i<=n-1;i++)   {    /// X...i i+1...X    ans+=sum1[i]*sum2[i+1];   }    cout<<ans<<endl;  }          return 0; }