Valid Palindrome

Problem:

Given a string, determine if it is a palindrome, considering only alphanumeric characters and ignoring cases.

For example,
"A man, a plan, a canal: Panama" is a palindrome.
"race a car" is not a palindrome.

Note:
Have you consider that the string might be empty? This is a good question to ask during an interview.

For the purpose of this problem, we define empty string as valid palindrome.

Analysis:
答案里面是从两端到中间,而且剔除字符和空格没有用多余的function，真简洁。我的想法是从中间到两端。明显复杂一些。Anyway, 能自己写出来通过是好事。对自己的预期不能再低了，呵呵。
Solution:

1:  public static boolean isPalindrome(String s)  
2:       {  
3:            int j=0;  
4:            int k=0;  
5:            s=trimString(s.toLowerCase());  
6:            int mod=s.length()%2;  
7:            if(mod==0)  
8:            {  
9:                 j=s.length()/2-1;  
10:                 k=j+1;  
11:            }  
12:            else  
13:            {  
14:                 j=s.length()/2;  
15:                 k=j;  
16:            }  
17:            for(;j>=0;j--,k++)  
18:            {  
19:                 if(s.charAt(j)!= s.charAt(k))  
20:                      return false;  
21:            }  
22:            return true;  
23:       }  
24:       public static String trimString(String s)  
25:       {  
26:      StringBuilder sb= new StringBuilder();  
27:            for(char c: s.toCharArray())  
28:            {  
29:                 if((c>='a'&&c<='z')||(c>='0'&&c<='9') )  
30:                 {  
31:                      sb.append(c);  
32:                 }  
33:            }  
34:            return sb.toString();  
35:       }