[leedcode 125] 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.

public class Solution {
    //注意题目不考虑大小写，因此需要利用toLowerCase进行转换，定义一个函数判断字符是否是数值或字符，从头到尾开始向中间遍历
    public boolean isPalindrome(String s) {
        if(s==null||s.length()<=0) return true;
        s=s.toLowerCase();
        int left=0;
        int right=s.length()-1;
        while(left<right){
            while(left<right&&!isValid(s.charAt(left))){
                left++;
            }
            while(left<right&&!isValid(s.charAt(right))){
                right--;
            }
            if(left<right&&s.charAt(left)!=s.charAt(right))return false;
            left++;
            right--;

        }
        return true;
    }
    public boolean isValid(char a){
        if(a>=‘a‘&&a<=‘z‘||a>=‘0‘&&a<=‘9‘)return true;
        return false;
    }
}