C :

#include <stdio.h>

#define BASE 10

int numberroot(int n)
{
    int temp;

    while(n >= BASE) {
        temp = n;

        n = 0;
        while(temp) {
            n += temp % BASE;
            temp /= BASE;
        }
    }

    return n;
}

int main(void)
{
    int n;

    scanf("%d", &n);

    printf("%d\n", numberroot(n));

    return 0;
}

C++ :

#include <bits/stdc++.h>
using namespace std;
 
int sum(int n){
    int s = 0;
    while(n != 0){
        s = s + n % 10;
        n = n / 10;
    }
    return s;
}
 
int main(){
    int n;
    cin>>n;
    int s = sum(n);
    while(s >= 10){
    	s = sum(s);
	}
	cout<<s<<endl;
}

Java :

import java.util.*;
public class Main{
	public static int getS(int n){
		int i,t=n;
		while(t>=10){
			i=0;
			n=t;
			while(n>0){
				i+=n%10;
				n/=10;
			}
			t=i;
		}
		return t;	
	}



	public static void main(String args[]){
		Scanner sc=new Scanner(System.in);
		int n=sc.nextInt();
		System.out.print(getS(n));
	}
}

Python :

n = int(input())
for i in range (1, 10):
        if n % 9 == i:
                print(i)