SumDigits.java

/*
Given a non-negative int n, return the sum of its digits recursively (no loops). Note that mod (%) by 10 yields the rightmost digit (126 % 10 is 6), while divide (/) by 10 removes the rightmost digit (126 / 10 is 12).
sumDigits(126) → 9
sumDigits(49) → 13
sumDigits(12) → 3
*/

public int triangle(int rows) {
  if(rows==0) return 0;
  return rows+triangle(rows-1);
}