Valid Palindrome

Valid Palindrome

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Valid Palindrome is a Leetcode problem that presents this question:

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.

Notably, the input can contain symbols that you need to strip out first. I did this using the replaceAll() function, but you could also do this using two pointers.

String lowerCased = s.toLowerCase();
String strippedString = lowerCased.replaceAll("[^a-z0-9]", "");

You might approach this problem by reversing the string and then checking for equality:

StringBuilder sb = new StringBuilder(strippedString);
String reversed = sb.reverse().toString();
boolean isPalindrome = strippedString.equals(reversed);
return isPalindrome;

This is a “clever” solution that shows familiarity with the language, but chances are your interviewer would follow this up by asking you to write out the logic yourself.

One approach is to use a for loop to check each character against its mirror:

class Solution {
  public boolean isPalindrome(String s) {

    String lowerCased = s.toLowerCase();
    String strippedString = lowerCased.replaceAll("[^a-z0-9]", "");

    for (int i = 0; i < strippedString.length() / 2; i++) {
      char leftChar = strippedString.charAt(i);
      char rightChar = strippedString.charAt(strippedString.length() - 1 - i);
      if(leftChar != rightChar){
        return false;

    return true;

Follow ups:

  • Return true if s contains a palindrome
  • Return true if s can be rearranged into a palindrome
  • Return the longest palindrome substring of s

Arrays and Strings Examples


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