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Given a non-negative integer num, repeatedly add all its digits until the result has only one digit.
For example:
Given num = 38, the process is like: 3 + 8 = 11, 1 + 1 = 2. Since 2 has only one digit, return it.
Solution. We need to refer to this wiki digital root. Basically, the result is num mod 9. But we need to exclude 2 exceptions:
1. When num is 0, we need to return 0.
2. When mod is 0, we need to return 0.
In this way, easiest one should be 
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