Write a program to find sum of all integers of a given number such that sum is always a single digit.
For instance if the given number is 6789, then answer should be 6+7+8+9 = 30 and then 3+0 =3.
A straight forward logic is to write a recursive function which calculates sum of digits until the it is less than 10. But there an efficient logic than this. Here is that
1. Calculate mod(input_integer,9)
2. if the result is 0 then
sum of digits reduced to single digit is 9
value returned in step 1 is the sum of digits of input integer reduced to 9.