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=1<<30;

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=0;
    while(x>0)
    {
        ans+=y[x];
        x-=lowbit(x);
    }
    return ans;
}

int main()
{
    scanf("%d",&n);
    for(int i=1; i<=n; i++)
    {
        scanf("%lld",&a[i]);
        xx[i]=a[i];
    }
    sort(xx+1,xx+1+n);
    int cnt=1;
    for(int i=2; i<=n; i++)
    {
        if(xx[i-1]!=xx[i])
        {
            xx[++cnt]=xx[i];
        }
    }
    for(int i=1; i<=n; i++)
    {
        int pos=lower_bound(xx+1,xx+cnt+1,a[i])-xx;
        vis1[pos]++;
        b[i]=vis1[pos];
    }
    for(int i=n; i>=1; i--)
    {
        int pos=lower_bound(xx+1,xx+cnt+1,a[i])-xx;
        vis2[pos]++;
        c[i]=vis2[pos];
    }
    ll ans=0;
    for(int i=n; i>=1; i--)
    {
        ans+=Getsum(b[i]-1);
        Insert(c[i],1);
    }
    printf("%I64d
",ans);
    return 0;
}