Description： Given a string, determine if it is a palindrome, considering only alphanumeric characters and ignoring cases.

Note: For the purpose of this problem, we define empty string as valid palindrome.

Example 1:

Input: "A man, a plan, a canal: Panama"
Output: true

Example 2:

Input: "race a car"
Output: false
 

題意：判斷一個字串是否是迴文，忽略除字母外的其他字元，並且對大小寫不敏感；

解法：可以從字串的首尾處開始向中間遍歷，找到出現的字母或者數字，判斷兩個字元是否相等；

Java

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