static int beautifulDays(int i, int j, int k) { int r=0,count=0;

    int l=Math.abs(i-j)+1;
    int[] ar=new int[l];
    int[] ar1=new int[l];
    int[] sum=new int[l];
    int n=i;

    for(int m=0;m<ar.length;m++)
    {
        ar[m]=i;
        i++;
        ar1[m]=n;
        n++;
        sum[m]=0;  
        while(ar[m]>0)
        {
            r=ar[m]%10;
            sum[m]=sum[m]*10 + r;
            ar[m]=ar[m]/10;

        }
        if(((Math.abs(ar1[m]-sum[m]))%k)==0)
            count++;
    }
        return count;
}

Performed similar logic but without String builder class.