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;
}