[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.

[Solution].java

public class Solution {
    public boolean isPalindrome(String s) {
        // Start typing your Java solution below
        // DO NOT write main() function
        if(s == null || s.length() < 1) return true;
        s = s.toLowerCase();
        int begin = 0;
        int end = s.length() - 1;
        while(begin < end){
            while(begin < s.length() && !isLetter(s.charAt(begin))) begin++;
        	while(end >= 0 && !isLetter(s.charAt(end))) end--;
        	if(begin > end) break;
        	if(s.charAt(begin) != s.charAt(end)) return false;
        	begin++;
        	end--;
        }
        return true;
    }

    public boolean isLetter(char c){
    	if(c <= 'z' && c >= 'a') return true;
    	else if(c <= '9' && c >= '0') return true;
    	return false;
    }
}