Valid Palindrome

A phrase is a palindrome if, after converting all uppercase letters into lowercase letters and removing all non-alphanumeric characters, it reads the same forward and backward. Alphanumeric characters include letters and numbers.

Given a string s, return true if it is a palindrome, or false otherwise.

Example 1:

Input: s = "A man, a plan, a canal: Panama"
Output: true
Explanation: "amanaplanacanalpanama" is a palindrome.
Example 2:

Input: s = "race a car"
Output: false
Explanation: "raceacar" is not a palindrome.
Example 3:

Input: s = " "
Output: true
Explanation: s is an empty string "" after removing non-alphanumeric characters.
Since an empty string reads the same forward and backward, it is a palindrome.
Constraints:

1 <= s.length <= 2 * 105
s consists only of printable ASCII characters.
 

思路一：轉小寫，去除非法字元，最後對比

public static boolean isPalindrome(String s) {
    s = s.toLowerCase();
    s = s.replaceAll("[^a-z\\d]", "");

    int i = 0;
    int j = s.length() - 1;

    while (i < j) {
        if (s.charAt(i++) != s.charAt(j--)) return false;
    }

    return true;
}