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319. Get the Sum of Digits in a number till it become a single digit
Objective  Given a number, Write a program to get the sum of digits in a number till it become a single digit.
Example:
N = 999 > 9+9+9 = 27> 2+7 = 9 N = 789 > 7+8+9= 24> 2+4 = 6
Approach:
Recursion
 Find the sum of all digits.
 Make a recursive call with the sum calculated in step 1.
 If the number is less than 10, return the number.
 See the code and click on the run code button for more understanding.
Output:
Sum of digits in a number 12345 till it become a single digit: 6 Sum of digits in the number 999 till it becomes a single digit: 9
Tricky Approach:
 If number is 0, return 0.
 Find remainder of number with 9. (number%9).
 If remainder is 0, return 9 else return the remainder.
Output:
Sum of digits in a number 12345 till it become a single digit: 6 Sum of digits in a number 999 till it become a single digit: 9 Sum of digits in a number 111 till it become a single digit: 3