Write digit_sum(n) that returns the sum of the decimal digits of the integer n. Ignore the sign: the digits of -905 are 9, 0, 5.
Examples
Input: n = 1234
Output: 10
Input: n = -905
Output: 14
Input: n = 0
Output: 0
Constraints
-10**18 <= n <= 10**18
Goals
- Peel off digits with `% 10` and `// 10`
- Handle zero and negative inputs