`//Java solution with recursion:-

public static int superDigit(String n, int k) { if(n.length()==1) return Integer.parseInt(n);

long sum = 0;
int i =0;
int[] digitArr = new int[n.length()];

while(i<n.length()){
    digitArr[i]=Integer.parseInt(n.charAt(i)+"");
    i++;
}

for(int j=0; j<digitArr.length;j++){
    if(digitArr[j]!=0)
        sum+=digitArr[j]*k;
}
//Calling the function recursively with n = String value of sum 
// and k = 1 as there is no repition required
return superDigit(String.valueOf(sum), 1); 
}

`