Valid Palindrome - Python - Leetcode 125 - Easy

 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.

 

Constraints:

• 1 <= s.length <= 2 * 105
• s consists only of printable ASCII characters.

Solution :

1st Way :

class Solution:
    def isPalindrome(self, s: str) -> bool:
        ns = ""
        for c in s:
            if c.isalnum():
                ns += c.lower()
        return ns == ns[::-1]

2nd Way

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):
        return (ord('A') <= ord(c) <= ord('Z') or
                ord('a') <= ord(c) <= ord('z') or
                ord('0') <= ord(c) <= ord('9'))

Explaination :