leetcode-go/leetcode/easy/258.add-digits.go

func addDigits(num int) int {
	if num == 0 { // If sum of digits is divisable by 9, then num is divisable by 9.
	// In fact, (num % 9) == [sum of digits] % 9
		return 0
	} else if n := num % 9; n == 0 {
		return 9
	} else {
		return n
	}
}