add-digits.go

package main

import (
	"fmt"
)

func addDigits(num int) int {
	for num >= 10 {
		sum := 0
		for ; num > 0; num /= 10 {
			sum += num % 10
		}
		num = sum
	}
	return num
}

func main() {
	fmt.Println(addDigits(10))
}