125. 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

Approach

1. Initializing two pointer left and right point to the left-most side and the right-most
2. Moving two pointers in and compare two pointers if one case does not match then know that string is not a palindrom

def isPalindrome(self, s: str) -> bool:
    import re
    
    s = re.sub("[\W_]+", "", s.lower())
    left = 0
    right = len(s) - 1
    
    while right > left:
        if s[left] != s[right]:
            return False
        right -= 1
        left += 1
    return True    
    

Complexity

• Time complexity: O(n)
• Space complexity: O(1)