Finding sum of digits of a number until sum becomes single digit

Given a number n we need to find the sum of it's digit until it becomes single digit and expected time complexity O(1).

Input:-  12345

Output-   6


Solution 1:- Brute Force

import java.util.*;

class GFG {
    static int getSum(int n)
    {
        int sum=0;
        while(n>0||sum>9)
        {
            if(n==0)
            {
                n=sum;
                sum=0;
            }
           
                sum+=n%10;
                n=n/10;
           
        }
        return sum;
    }
    public static void main (String[] args) {
        Scanner sc=new Scanner(System.in);
        int n=sc.nextInt();
        System.out.println(getSum(n));
    }
}


Solution 2:- Best Way


import java.util.*;
class GFG {
    static int getSum(int n)
    {
        if(n==0)
          return n;
        return (n%9==0?9:n%9); 
    }
    public static void main (String[] args) {
        Scanner sc=new Scanner(System.in);
        int n=sc.nextInt();
        System.out.println(getSum(n));
    }
}

Time Complexity- O(1)