Hi there,
Where is my mistake? Is my code right?

package com.codegym.task.task30.task3010;
/*
Smallest possible radix
*/
import java.math.BigInteger;
public class Solution
{
public static void main(String[] args)
{
//write your code here
// args = new String[]{" EZ"};
if (isValidString(args[0]))
{
System.out.println(getMinimumRadix(args[0]));
}
else {
System.out.println("Invalid");
}
}
// Checks if all chars in args[0] are letters o number
private static boolean isValidString(String str)
{
boolean isValid = false;
char[] ch = str.toCharArray();
for(char c : ch){
isValid = Character.isLetterOrDigit(c);
if(!isValid)
{
break;
}
}
return isValid;
}
private static String getMinimumRadix(String str)
{
final int LOWER_RADIX = 2;
final int UPPER_RADIX = 36;
for(int radix = LOWER_RADIX; radix <= UPPER_RADIX; radix++)
{
try
{
BigInteger number = new BigInteger(str, radix);
return String.valueOf(radix);
}
catch(Exception e)
{
}
}
return null;
}
}

"15h7D"The highest char is 'h', which is the 7th (a= 0, h = 7) letter of the alphabet. That will mean the smallest radix would be 18 (start at 10 for the digits 0-9, then add 7 for 'h', then add 1:10 + 7 + 1 = 18). #2"4365"The largest char is '6' so the answer is 6+1 = 7 Just be sure to count alphanumeric digits the same regardless of their case ('t' is equal to 'T' as a numeral digit).