REVSTR Problem code: NF03 - NZEC appearing for the Java code

Hi there,
While submitting Java code for the problem - http://www.codechef.com/problems/NF03/, the NZEC is appearing. It seems that somewhere Exception is thrown. Could you help me finding the problem?

The code:
package magic.labs.algorithms;

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;

class ReverseStr {

public static void main(String[] args)  {
    System.out.println(result());
}

private static String result() {
    BufferedReader obj=new BufferedReader(new InputStreamReader(System.in));
    String s = null;        
	
    try {
        s = obj.readLine();
    } catch (IOException e) {
        ;
    }
    if(s == null) {
        return null;
    }
        
    char[] string = new char[s.length()];
    for(int i=0;i<string.length;i++)
        string[i] = s.charAt(i);
    int b=0;
    for(int i=0;i<string.length;i++) {
        if ((string[i] < 'a' || string[i] > 'z') && (string[i] < 'A' || string[i] > 'Z') && (string[i] < '0' || string[i] > '9')) {        
            reverse(string,b,i);
            b=i+1;
        }
    }
    reverse(string,b,string.length);
    StringBuilder str = new StringBuilder();
    for(int i=0;i<string.length;i++) {
        str.append(string[i]);
    }
    return str.toString();

}

private static void reverse(char[] s, int begin, int end) {
    int len = end - begin;
    for (int i = begin; i < begin + len / 2; i++) {
        char t = s[i];
        s[i] = s[begin + end - i - 1];
        s[begin + end - i - 1] = t;
    }
}

}

I’m sorry for clumsy code.

Thanks in advance.

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