125 Valid Palindrome - Easy

Problem:

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.

Problem Analysis:

interviewer may not allow us to use built-in functions like char.isalnum(), we can implement our own alnum by converting the characters to ascii characters
initialize 2 pointers, one at each side of the side and increment them towards each other and perform a character check using while loop
take note to move pointer if the current pointer is at a non-alphanumeric character

Solutions:

Solution 1

class Solution:
    def isPalindrome(self, s: str) -> bool:
        l, r = 0, len(s) - 1

        while l < r:
            while l < r and not self.alphaNum(s[l]):
                l +=1
            while r > l and not self.alphaNum(s[r]):
                r -=1
            if s[l].lower() != s[r].lower():
                return False
            l, r = l + 1, r - 1
        return True
    
    def alphaNum(self,c):
        # implement own alphanum func
        # The ord() function returns the number representing the unicode code of a specified character.
        return (ord('A') <= ord(c) <= ord('Z') or
            ord('a') <= ord(c) <= ord('z') or
            ord('0') <= ord(c) <= ord('9'))

Solution 2

class Solution:
    def isPalindrome(self, s: str) -> bool:
        cleaned_string = ''.join(char.lower() for char in s if char.isalnum())
        return cleaned_string == cleaned_string[::-1]