Sum Digits Until One

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)

```js
function sumDigits(num) {
	return num;
};
```