Leetcode: Valid Palindrome

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.

 1 public class Solution {
 2     public boolean isPalindrome(String s) {
 3         if (s == null || s.length() == 0) {
 4             return true;
 5         }
 6         int l = 0;
 7         int r = s.length() - 1;
 8         while (l < r) {
 9             while (l<s.length() && !isValidChar(s.charAt(l))) {
10                 l++;
11             }
12             while (r>0 && !isValidChar(s.charAt(r))) {
13                 r--;
14             }
15             if (l < r) {
16                 if (isSame(s.charAt(l), s.charAt(r))) {
17                     l++;
18                     r--;
19                 }
20                 else return false;
21             }
22         }
23         return true;
24     }
25     
26     public boolean isValidChar(char c) {
27         if ((c>=‘a‘ && c<=‘z‘) || (c>=‘A‘ && c<=‘Z‘) || (c>=‘0‘ && c<=‘9‘))
28             return true;
29         else return false;
30     }
31     
32     public boolean isSame(char s1, char s2) {
33         if (s1>=‘A‘ && s1<=‘Z‘) {
34             s1 = (char)(s1 - ‘A‘ + ‘a‘);
35         }
36         if (s2>=‘A‘ && s2<=‘Z‘) {
37             s2 = (char)(s2 - ‘A‘ + ‘a‘);
38         }
39         if (s1 == s2) {
40             return true;
41         }
42         else return false;
43     }
44 }