567C - Geometric Progression - CodeForces Solution

#include <bits/stdc++.h>
using namespace std;
#define int long long int

int32_t main()
{
    int n, k;
    cin >> n >> k;
    int arr[n];
    map<int, int> pre, back;
    for (int i = 0; i < n; i++)
    {
        cin >> arr[i];
        back[arr[i]]++;
    }

    int ans = 0;

    for (int i = 0; i < n; i++)
    {
        back[arr[i]]--;
        if (arr[i] % k == 0)
        {
            ans += (pre[arr[i] / k] * back[arr[i] * k]);
        }
        pre[arr[i]]++;
    }

    cout << ans << endl;
}