A Harshad number (also called a Niven number) is a positive integer that is divisible by the sum of its digits. For example, 18 is a Harshad number because its digit sum is 1+8=9, and 18 is divisible by 9. Return false for numbers less than or equal to 0.
Examples:
Input: n = 18
Output: true
Explanation: Digit sum = 1 + 8 = 9, and 18 / 9 = 2 (divisible)
Input: n = 21
Output: true
Explanation: Digit sum = 2 + 1 = 3, and 21 / 3 = 7 (divisible)
Input: n = 19
Output: false
Explanation: Digit sum = 1 + 9 = 10, and 19 is not divisible by 10
Input: n = 1
Output: true
Explanation: Digit sum = 1, and 1 / 1 = 1 (divisible)
Input: n = 12
Output: true
Explanation: Digit sum = 1 + 2 = 3, and 12 / 3 = 4 (divisible)
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