public static long sumOfGroup(int k) {

 long ele=(k*(k-1))+1;
long sum=(k*(2*ele+(k-1)*2))/2;
return sum;



}

def sumOfGroup(k):
        return k**3

seriuosly, Why does "k*k*K" solution work !?

I mean what the math explaination behind this?

// Return the sum of the elements of the k'th group.
long long int x=(k*(k-1)+1);
long long int sum=x;
for(int i=0;i<k-1;i++){
    x=x+2;
    sum+=x;
}

return sum; }https://)

For th group, how many number were used in previous groups?

So, the first number in th group will be th odd number.

th odd number is given as . So, the first number in th group will be

Now, the group is an AP with first term given above and common difference (odd numbers).

In C++, return 1L * k * k * k.