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125. Valid Palindrome


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: falseSolution


class Solution {
    public static boolean isPalindrome(String s) {
        if (s == null) return false;
        if (s.length() == 0) return true;

        s = s.toLowerCase().trim();
        int i = 0;
        int j = s.length() - 1;
        while (i < j) {
            while (i < j && !Character.isLetterOrDigit(s.charAt(i))) i++ ;
            while (i < j && !Character.isLetterOrDigit(s.charAt(j))) j--;
            if (s.charAt(i++) != s.charAt(j--)) return false;
        return true;

    public static void main(String[] args) {
        isPalindrome("A man, a plan, a canal: Panama");

Execution time: 18 ms, defeating 34.61% of users in Valid Palindrome’s Java submission

Memory consumption: 38.3 MB, defeating 78.33% of users in Valid Palindrome’s Java submission

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