codeforces D. Pashmak and Parmida's problem

http://codeforces.com/contest/459/problem/D

题意：给你n个数，然后统计多少组（i,j）使得f(1,i,ai)>f(j,n,aj);

思路：先从左往右统计f(1,i,ai)记录在b[i],然后从右往左统计f(j,n,aj)记录在c[i],然后利用树状数组统计个数即可。

 #include <cstdio>
 #include <iostream>
 #include <cstring>
 #include <cmath>
 #include <queue>
 #include <map>
 #include <algorithm>
 #define maxn 1000010
 #define ll long long
 using namespace std;
 const int inf=<<;

 int n,m;
 ll a[maxn];
 ll b[maxn];
 ll c[maxn];
 ll y[maxn];
 ll xx[maxn];
 ll vis1[maxn],vis2[maxn];

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

 void Insert(int x,ll val)
 {
     while(x<maxn)
     {
         y[x]+=val;
         x+=lowbit(x);
     }
 }

 ll Getsum(int x)
 {
     ll ans=;
     while(x>)
     {
         ans+=y[x];
         x-=lowbit(x);
     }
     return ans;
 }

 int main()
 {
     scanf("%d",&n);
     for(int i=; i<=n; i++)
     {
         scanf("%lld",&a[i]);
         xx[i]=a[i];
     }
     sort(xx+,xx++n);
     int cnt=;
     for(int i=; i<=n; i++)
     {
         if(xx[i-]!=xx[i])
         {
             xx[++cnt]=xx[i];
         }
     }
     for(int i=; i<=n; i++)
     {
         int pos=lower_bound(xx+,xx+cnt+,a[i])-xx;
         vis1[pos]++;
         b[i]=vis1[pos];
     }
     for(int i=n; i>=; i--)
     {
         int pos=lower_bound(xx+,xx+cnt+,a[i])-xx;
         vis2[pos]++;
         c[i]=vis2[pos];
     }
     ll ans=;
     for(int i=n; i>=; i--)
     {
         ans+=Getsum(b[i]-);
         Insert(c[i],);
     }
     printf("%I64d\n",ans);
     return ;
 }