Given a non-negative integer num, repeatedly add all its digits until the result has only one digit.

For example:

Given num = 38, the process is like: 3 + 8 = 11, 1 + 1 = 2. Since 2 has only one digit, return it.

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public class Solution {
    public int addDigits(int num) {
        while (num >= 10){
            int sum = 0;
            while (num > 0){
                sum += num % 10;
                num /= 10;
            }
            num = sum;
        }
        return num;
    }
}

Without any loop/recursion in O(1) runtime

See explanaton on Wikipedia or discussion on Leetcode

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public class Solution {
    public int addDigits(int num) {
        return 1 + (num - 1) % 9;
    }
}