125. Valid Palindrome

Description

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

valid-palindrome.py

class Solution:
    def isPalindrome(self, s: str) -> bool:
        beg, end = 0, len(s) - 1
        while beg < end:
            while not s[beg].isalnum() and beg < end: beg += 1
            while not s[end].isalnum() and beg < end: end -= 1
            if s[beg] == s[end] or s[beg].upper() == s[end].upper():
                beg, end = beg + 1, end - 1
            else:
                return False
        return True