Good afternoon! Here's our prompt for today.

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:

1// start with 49
24 + 9 = 13
3
4// since the previous addition was 13,
5// move onto summing 13's digits
61 + 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)

Try to solve this here or in Interactive Mode.

How do I practice this challenge?

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var assert = require('assert');

​

function sumDigits(num) {

  return num;

}

​

try {

  assert.equal(sumDigits(1), 1);

​

  console.log('PASSED: ' + 'sumDigits(1) should return 1');

} catch (err) {

  console.log(err);

}

​

try {

  assert.equal(sumDigits(49), 4);

​

  console.log('PASSED: ' + 'sumDigits(49) should return 4');

} catch (err) {

  console.log(err);

}

​

try {

  assert.equal(sumDigits(439230), 3);

​

  console.log('PASSED: ' + 'sumDigits(439230) should return 3');

} catch (err) {

  console.log(err);

}

Here's our guided, illustrated walk-through.

How do I use this guide?