125. 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.


class Solution {
    public boolean isPalindrome(String s) {
        // 判断字符串是否为空，为空则返回 true
        if (s == null || s.length() == 0) {
            return true;
        }

        // 将字符串转换为小写字母并去除非字母数字字符
        String lowerCase = s.toLowerCase().replaceAll("[^a-z0-9]", "");
        int left = 0, right = lowerCase.length() - 1;

        // 使用双指针，从两端向中间比较字符是否相同
        while (left < right) {
            if (lowerCase.charAt(left) != lowerCase.charAt(right)) {
                return false;
            }
            left++;
            right--;
        }

        return true;
    }
}

注释：

判断字符串是否为空，如果为空则返回 true，因为空字符串是回文串。
将字符串转换为小写字母并去除非字母数字字符。
使用双指针，从两端向中间比较字符是否相同。如果有任意一个字符不同，则返回 false。
如果所有字符都相同，则返回 true。