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.

思路：双指针夹逼，判断首位对应的字母是否相同。Character.isLetterOrDigit这个method好好用！Reverse Words in String就可以用这个method和stack读出来words，然后pop出来。

public class Solution {
    public boolean isPalindrome(String s) {
        int i=0,j=s.length()-1;
        while(i<j)
        {
            while(i<j&&!Character.isLetterOrDigit(s.charAt(i)))
            {
                i++;
            }
            while(i<j&&!Character.isLetterOrDigit(s.charAt(j)))
            {
                j--;
            }
            if(Character.toLowerCase(s.charAt(i))!=Character.toLowerCase(s.charAt(j)))
            {
                return false;
            }
            i++;
            j--;
            }
            return true;
        
    }
}