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.

一刷
题解：思路很简单，但是有很多细节需要注意

1. 字母和数字都是有效的， 且a和A相等。所以要统一转为up或low case然后再用相等条件。
2. 注意在lo++, hi--的时候不能超出边界所以.charAt之前要边界判断。
3. 注意存在函数Character.isLetterOrDigit

public class Solution {
    public boolean isPalindrome(String s) {
        int lo = 0, hi = s.length()-1;
        while(lo

二刷
two pointer