// https://www.hackerrank.com/challenges/java-interface class MyCalculator implements AdvancedArithmetic { public int divisorSum(int n) { int sum = 1; if (n != 1) sum += n; // since we know divisible by 1 and itself (unless 1) for(int i = 2; i <= (int)Math.sqrt(n); i++) { if(n % i == 0) { sum += i; int d = n/i; if(d != i) sum+=d; } } return sum; } }