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.

題意：判斷字符串是不是有效回文串，忽略掉標點，只考慮字母和數(shù)字，字母忽略大小寫。

思路：水題，上代碼

java代碼：

public boolean isPalindrome(String s) {
        s = s.toLowerCase();
        char[] ss = s.toCharArray();
        List<Character> list = new ArrayList<>();
        for (int i = 0; i < ss.length; i++) {
            if ((ss[i] >= 'a' && ss[i] <= 'z') || (ss[i] <= 'Z' && ss[i] >= 'A') || (ss[i] <= '9') && ss[i] >= '0') {
                list.add(ss[i]);
            }
        }
        Character[] sss = new Character[list.size()];
        list.toArray(sss);
        for (int i = 0; i < sss.length / 2; i++) {
            if (sss[i] != sss[sss.length - i - 1]) {
                return false;
            }
        }
        return true;
    }