Hi, here's your problem today. This problem was recently asked by Amazon:

Given a number like 159, add the digits repeatedly until you get a single number.

For instance, 1 + 5 + 9 = 15. 1 + 5 = 6.

So the answer is 6.

```
class Solution(object):
# Fill this in.
print(Solution().addDigits(159))
# 1 + 5 + 9 = 15
# 1 + 5 = 6
# 6
```