Valid Palindrome

Problem Source: Leetcode

Problem 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.

tests

def test(fn):
    s = "A man, a plan, a canal: Panama"
    assert fn(s)
    s = "race a car"
    assert not fn(s)
    s = " "
    assert fn(s)

Solution

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

def isPalindrome(s: str) -> bool:
    lp, rp = 0, len(s)-1

    while lp < rp:        
        # Ignore non-alphanumeric characters by moving pointer
        # Alternative to if statements is filter(lambda x: x.isalnum(), s) prior to loop
        if not s[lp].isalnum():
            lp += 1
            continue
        if not s[rp].isalnum():
            rp -= 1
            continue        

        # Return and exit at first failure
        if s[lp].lower() != s[rp].lower():
            return False

        # Move to next characters
        lp += 1
        rp -= 1

    # If no failures, then it's a palindrome
    return True

test(isPalindrome)