258.md

258. Add Digits

Description

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.

Follow up: Could you do it without any loop/recursion in O(1) runtime?

Thinking Process

1. Approach 1, the easiest way is just to iterate until num < 10
2. Approach 2,

Code

Approach 1

public class Solution {
    public int addDigits(int num) {
        while(num > 9){
            num = nextIteration(num);
        }
        return num;
    }
    int nextIteration(int num){
        int sum = 0;
        while(num != 0){
            sum += (num % 10);
            num /= 10;
        }
        return sum;
    }
}

Approach 2