int beautifulTriplets(int d, vector<int> arr) {
    // bucket
    int count(0);
    int n = arr.size();
    int b_size = 20001;
    vector<int> bucket(b_size);
    for(int i(0);i<n;++i){
        ++bucket[arr[i]];
    }

    for(int i(0);i<n;++i){
        int num_j = arr[i]+d;
        int num_k = arr[i] + 2 *d;
        if(num_j>=b_size || num_k >= b_size) continue;

        count += bucket[num_j] * bucket[num_k];
    }
    return count;
}