Here is my O(n) c++ solution, you can watch the implementation here : https://youtu.be/vLD3N79nLSE

int beautifulTriplets(int d, vector<int> arr) {
    map<int, int> mp;
    int result = 0;
    for (int a : arr) {
        mp[a] += 1;
        result += mp[a-d]*mp[a-2*d];
    }
    return result;
}

function beautifulTriplets(d, arr) {

let hash = {}
let counter = 0
for (const n of arr) {
    hash[n] = !hash[n] ? 1 : hash[n] + 1;
}

for (const k in hash) {
    const sec = Number(k) + d;
    const third = sec + d;

    if (hash[sec] && hash[third]) counter += Math.max(hash[k], hash[sec], hash[third])
}
return counter

}

public static int beautifulTriplets(int d, List arr) { // Write your code here Set set=new HashSet<>(); for(int i:arr) set.add(i); int cnt=0,c,e; int l=arr.size(); for(int i=0;i

    }  

return cnt; }

include

include

using namespace std;

string ltrim(const string &); string rtrim(const string &); vector split(const string &);

int beautifulTriplets(int d, vector arr) { int triplets=0;

for(int i=0;i<arr.size();i++){
    for(int j=i+1;j<arr.size();j++){
        if (arr[j]-arr[i]==d) {
        for(int k=j+1;k<arr.size();k++){
            if(arr[k]-arr[j]==d){
               triplets++;
               break;
        }
        }
        }
    }
}

return triplets; }

int main() { ofstream fout(getenv("OUTPUT_PATH"));

string first_multiple_input_temp;
getline(cin, first_multiple_input_temp);

vector<string> first_multiple_input = split(rtrim(first_multiple_input_temp));

int n = stoi(first_multiple_input[0]);

int d = stoi(first_multiple_input[1]);

string arr_temp_temp;
getline(cin, arr_temp_temp);

vector<string> arr_temp = split(rtrim(arr_temp_temp));

vector<int> arr(n);

for (int i = 0; i < n; i++) {
    int arr_item = stoi(arr_temp[i]);

    arr[i] = arr_item;
}

int result = beautifulTriplets(d, arr);

fout << result << "\n";

fout.close();

return 0;

}

string ltrim(const string &str) { string s(str);

s.erase(
    s.begin(),
    find_if(s.begin(), s.end(), not1(ptr_fun<int, int>(isspace)))
);

return s;

}

string rtrim(const string &str) { string s(str);

s.erase(
    find_if(s.rbegin(), s.rend(), not1(ptr_fun<int, int>(isspace))).base(),
    s.end()
);

return s;

}

vector split(const string &str) { vector tokens;

string::size_type start = 0;
string::size_type end = 0;

while ((end = str.find(" ", start)) != string::npos) {
    tokens.push_back(str.substr(start, end - start));

    start = end + 1;
}

tokens.push_back(str.substr(start));

return tokens;

}