Is a Valid Palindrome with Java

The challenge

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


  • s consists only of printable ASCII characters.

The solution in Java

class Solution {
    // take in a String
    public boolean isPalindrome(String s) {
        // remove all non-alphanumeric characters and lowercase everything
        s = s.replaceAll("\\P{Alnum}", "").toLowerCase();
        // loop through the String
        for (int i=0; i<s.length(); i++) {
            // if the beginning and last characters don't match, return false
            // also walk in both directions and do the same
            if (s.charAt(i)!=s.charAt(s.length()-i-1))
                return false;
        // return true if we get here
        return true;
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