Given an integer num, repeatedly add all its digits until the result has only one digit, and return it.

Example 1:

Input: num = 38
Output: 2

Explanation: The process is
38 --> 3 + 8 --> 11
11 --> 1 + 1 --> 2
Since 2 has only one digit, return it.

Example 2:

Input: num = 0
Output: 0

def add_digits(num)
  while num > 9
      num = num.to_s.chars.sum{ |s| s.to_i } 
  end
  num
end

也可以試著練習換換不同寫法

def add_digits(num)
  while num > 9
      num = num.to_s.chars.sum(&:to_i)
  end
  num
end

參考資料：Ruby - &: And &method With To_proc