title: increment an integer split in an array recursion
body: <p>Imagine you have integers split into arrays like 100 -> [1, 0, 0]</p>
  
  <p>How do you write a <strong>recursive</strong> function that increments the long integer. eg incr([9, 9]) -> [1, 0, 0]?</p>
  
  <p>I know how to do it non recursively.</p>