Question

Method return should be like if entered a number, suppose 345, then output should be 3+4+5=12 –> 1+2 = 3. what i am doing wrong here?

public class DigitSum
 {
    int  Sum=0;

    public int compute( int MethParam )
    {
        int rem = MethParam%10; 
        Sum+=rem;        

        MethParam = MethParam/10; 
        if(MethParam>10)
            compute(MethParam);

        return Sum+MethParam;  
    }

  public static void main(String[] args)
  {
    DigitSum ds  = new DigitSum();
    System.out.println(ds.compute(435));
  }
}

Answer 1

O(1) Algo for sum Of digits :

Taking the modulo 9 of any number will return the sum of digits of that number until a single digit number is obtained.

If the number is a multiple of 9, then the sum will will be 9

one liner :

public int sumDigit(int n){
    return (n%9 == 0 && n != 0) ? 9 : n%9;
}

Alternate implementation :

public int sumDigit(int n){

      int sum = n % 9;
      if(sum == 0){
          if(n > 0)
               return 9;
      }
      return sum;
}