D. Pashmak and Parmida's problem 树状数组

Parmida is a clever girl and she wants to participate in Olympiads this year. Of course she wants her partner to be clever too (although he’s not)! Parmida has prepared the following test problem for Pashmak.

There is a sequence a that consists of n integers a1, a2, …, an. Let’s denote f(l, r, x) the number of indices k such that: l ≤ k ≤ r and ak = x. His task is to calculate the number of pairs of indicies i, j (1 ≤ i < j ≤ n) such that f(1, i, ai) > f(j, n, aj).

Help Pashmak with the test.

Input
The first line of the input contains an integer n (1 ≤ n ≤ 106). The second line contains n space-separated integers a1, a2, …, an (1 ≤ ai ≤ 109).

Output
Print a single integer — the answer to the problem.

Examples
inputCopy
7
1 2 1 1 2 2 1
outputCopy
8
inputCopy
3
1 1 1
outputCopy
1
inputCopy
5
1 2 3 4 5
outputCopy
0

先预处理一下，然后逆序对搞一搞就好

#include<iostream>
#include<cstdio>
#include<algorithm>
#include<queue>
#include<string>
#include<cstring>
#include<set>
#include<queue>
#include<map>
using namespace std;
#define maxn 1000005
#define inf 0x3f3f3f3f
typedef long long ll;
typedef unsigned long long int ull;

typedef pair<int,int> pii;
int n;
int a[maxn];
map<int,int>mp;
int b[maxn],c[maxn];
void add(int x)
{
    for(;x<=n;x+=x&(-x))a[x]++;
}
ll query(int x)
{
    ll ans=0;
    for(;x;x-=x&(-x))ans+=a[x];
    return ans;
}

int main()
{
    ios::sync_with_stdio(false);
  //  int n;
    cin>>n;
    int i,j;
    for(i=1;i<=n;i++)cin>>a[i];
    for(i=1;i<=n;i++){
        mp[a[i]]++;
        b[i]=mp[a[i]];
    }
    mp.clear();
    for(i=n;i>=1;i--){
        mp[a[i]]++;
        c[i]=mp[a[i]];
    }
    memset(a,0,sizeof(a));
    ll res=0;
    for(i=n-1;i>=2;i--){
        add(c[i+1]);
        res+=query(b[i]-1);
    }
    cout<<res<<endl;
}