LintCode 有效回文串题目分析代码

public class Solution {
    /**
     * @param s A string
     * @return Whether the string is a valid palindrome
     */
    public boolean isPalindrome(String s) {
        // Write your code here
        if(s == null || s.length() == 0)
            return true;
        
        int front = 0;
        int end = s.length() - 1;
        
        while(front < end)
        {
            while(front < s.length() && !isvalid(s.charAt(front)))
                front++;
                
            if(front == s.length())
                return true;
            
            while(end > 0 && !isvalid(s.charAt(end)))
                end--;
            
            if(Character.toLowerCase(s.charAt(front)) != Character.toLowerCase(s.charAt(end)))
                return false;
            else
            {
                front++;
                end--;
            }
        }
        
        return true; 
    }

    private boolean isvalid (char c) {
        return Character.isLetter(c) || Character.isDigit(c);
    }
}