258. Add Digits

Easy
Given an integer num, repeatedly add all its digits until the result has only one digit, and return it.
Example 1:
Input: num = 38
Output:
2
Explanation:
The process is
38 --> 3 + 8 --> 11
11 --> 1 + 1 --> 2
Since 2 has only one digit, return it.
Example 2:
Input: num = 0
Output:
0
Constraints:
  • 0 <= num <= 231 - 1
Follow up: Could you do it without any loop/recursion in O(1) runtime?

解題

func addDigits(num int) int {
if num<10 { return num }
n := strconv.Itoa(num)
sum := num
for sum>=10 {
n = strconv.Itoa(sum)
sum = 0
for _, val :=range n {
sum += int(val) - '0'
}
}
return sum
}
官方理想的正解:
func addDigits(num int) int {
return 1 + (num - 1) % 9;
}