Count Triplets

counttriplets.cpp

// problem : https://www.hackerrank.com/challenges/count-triplets-1/problem
// solution : https://www.hackerrank.com/challenges/count-triplets-1/forum/comments/468507

long countTriplets(vector<long> arr, long r) {
    map<int,long> mp2, mp3;
    //mp2 to hold count of needed values after this one to complete 2nd part of triplet
    //mp3 to hold count of needed values to complete triplet

    long count = 0;
    for( long val : arr )
    {
        if ( mp3[val] > 0 )        //This value completes mp3[val] triplets
        {
            count += mp3[val];
        }

        if ( mp2[val] > 0 )        //This value is valid as 2° part of mp2[val] triplets
        {
            mp3[val*r] += mp2[val];
        }
        
        mp2[val*r]++;            //"Push-up" this value as possible triplet start
    }

    return count;
}