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.

Solution

We do what the question says: if we encounter a non-alphanumeric character, we remove it.

Otherwise, we cast each character to lowercase.

We then check if that is equal to its reverse.

class Solution:
    def isPalindrome(self, s: str) -> bool:
        s = [c for c in s.lower() if c.isalnum()]

        return s[::-1] == s

Table of Contents

Valid Palindrome

Solution