回文字符串

public class Solution {
   public boolean isPalindrome(String s) {
    if(s == null) return true;
    int start = 0;
    int end = s.length()-1;
    char l,r;
    while(start <= end){
        l = s.charAt(start);
        r = s.charAt(end);
        while(start < end && !isAlphanumeric(l)) {
           start++;
           l = s.charAt(start); 

        }
        while(start < end && !isAlphanumeric(r)){
            end--;
            r = s.charAt(end);
        } 
        if(start <= end){
            if(isDigit(l) && !isDigit(r) || !isDigit(l) && isDigit(r))
                return false;
            if(l != r && Math.abs(l-r) != 32)
                return false;
        }
        start++;
        end--;
    }
    return true;
}
private boolean isAlphanumeric(char c){
    if(c >= 'A' && c <= 'Z' || c >= 'a' && c <= 'z' || c >= '0' && c <= '9') return true;
    return false;
} 
private boolean isDigit(char c){
    if(c >= '0' && c <= '9') return true;
    return false;
}
}