thnks for explanation here's the code in c++ if anyone interested: hard time struck in defining a map with pair as a key tho ; }

struct pair_hash
{
        template <class T1, class T2>
        std::size_t operator() (const std::pair<T1, T2> &pair) const
        {
                return std::hash<T1>()(pair.first) ^ std::hash<T2>()(pair.second);
        }
};

// Complete the countTriplets function below.
long countTriplets(vector<long> arr, long r) {
        unordered_map<int, long> freq;
        long nxt,thrd;
        long count=0;

        // store 2 and 3 part of triplet as key
        unordered_map<pair<int, int> , long, pair_hash > pairs; 

        for(auto i=arr.rbegin(); i!=arr.rend(); i++ ){
                nxt = *i * r;               //next element of triplet

                thrd = *i *(r*r);           //3rd elemnt of the triplet
                count+= pairs[{nxt,thrd}];    //if we get 1st element of the triplet
                pairs[{*i, nxt}] += freq[nxt];   //if second elemnt of the triplet

                freq[*i]++;                  //in case 3rd elemnt
        }
        return count;
}