Recently I wrote this code to find the sum of all the digits of the number until the number turns into a one digit number. For example: number 567, we add all the digits and we get 5+6+7 = 18, now we add all the digits of 18 and we get 1+8 = 9. We can't add anymore digits because there is only one left and thus the result is 9.

Now I am having trouble figuring out the time complexity of this recursive code. Can anyone help?

int digSum(int n)
{
    if(n < 10) return n;
    return digSum(digSum(n/10) + n%10);
}