title: Fastest method for adding/summing the individual digit components of a number
body: <p>I saw a question on a math forum a while back where a person was discussing adding up the digits in a number over and over again until a single digit is achieved. (i.e. "362" would become "3+6+2" which would become "11"... then "11" would become "1+1" would would become "2" therefor "362" would return 2... I wrote some nice code to get an answer to this and posted it only to be outdone by a user who suggested that any number in modulo 9 is equal to this "infinite digit sum", I checked it an he was right... well almost right, if zero was returned you had to switch it out with a "9" but that was a very quick fix...</p>
  
  <p>362 = 3+6+2 = 11 = 1+1 = 2</p>
  
  <p>or...</p>
  
  <p>362%9 = 2</p>
  
  <p>Anways, the mod9 method works fantastic for infinitely adding the sum of the digits until you are left with just a single digit... but what about only doing it once (i.e. 362 would just return "11")... Can anyone think of fast algorithms?</p>