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Sum Digits Until One

Here is the interview question prompt, presented for reference.

We're provided a positive integer num. Can you write a method to repeatedly add all of its digits until the result has only one digit?

Here's an example: if the input was 49, we'd go through the following steps:

// start with 49
4 + 9 = 13

// since the previous addition was 13,
// move onto summing 13's digits
1 + 3 = 4

We would then return 4.

Constraints

  • Input will be in the range between 0 and 1000000000
  • Expected time complexity : O(log n)
  • Expected space complexity : O(1)

You can see the full challenge with visuals at this link.

Challenges • Asked about 4 years ago by Danaila Marian

jch2Xer Commented on Aug 01, 2019:

This is called the digital room problem or This is called the digital **root* problem?Great job on this site by the way, I am enjoying it! thank you!!

Jake from AlgoDaily Commented on Oct 03, 2019:

Definitely digital root :-) Glad you're enjoying it!

Danaila Marian Commented on Feb 11, 2020:

Hi, guys!
I have a question about "Sum Digits Until One" problem. When I run the code for all the test cases everything is perfect, but when I click 'RUN TESTS' button, it gives me an error:

4
('An error occurred!', NameError("name 'assertEqual' is not defined",))
('An error occurred!', NameError("name 'assertEqual' is not defined",))
('An error occurred!', NameError("name 'assertEqual' is not defined",))

Here is the code:

def sumDigits(n):
if n < 10:
return n
else :
s = 0
while (n > 0):
s += n % 10
n //= 10
return sumDigits(s)
print(sumDigits(49))

Some thoughts about what is wrong?

Al Commented on May 22, 2020:

Same problem here

Anonymous Commented on Sep 18, 2020:

The assertions in the tests for this are incorrect. For example, I am seeing the following: assert sum_digits(49) == 4

It should be sum_digits(49) == 13

Jake from AlgoDaily Commented on Sep 18, 2020:

This is incorrect. The question states "write a method to REPEATEDLY add all of its digits until the result has only one digit". You can see in the example that we later add 1 + 3 from 13 to get 4.

ryanmvazquez@gmail.com Commented on Jan 19, 2022:

I think the example here could be made a bit more clear. It isn't obvious from the problem statement and example that all of the digits in the number needed to be added first to provide the next sum.

One could alternatively interpret the problem statement as fibonnaci-esque - deriving the sum from the first digit and the "reduced" sum of the subsequent digits (which could potentially grow infinitely).

The intent could be easily disambiguated by providing an example with > 2 digits.

> for x = 195
> 1 + 9 + 5 = 15
> 1 + 5 = 6