Understanding the Problem

It is a palindrome problem with some extra steps to solve it. Some sentences are included in the test cases like s = "A man, a plan, a canal: Panama". It is only to be left with lower case alpha numerical characters, which will form as ”amanaplanacanalpanama” and this is a palindrome.

An empty string s = ““ is also considered as a palindrome.

Approach

It was nothing much to think personally, so I went with filtering the alpha numeric characters only. I wanted to go loop with for and check from both sides of the string.

Solution

class Solution:
    def isPalindrome(self, s: str) -> bool:
        new_s = ""
        for c in s:
            # if (c.lower() >= "a" and c.lower() <= "z") or (c.lower() >= "0" and c.lower() <= "9"):
            #     new_s += c.lower()
            if c.lower().isalnum():
                new_s += c.lower()

        if new_s == "":
            return True

        for i in range(len(new_s)//2):
            if new_s[i] != new_s[-(i+1)]:
                return False
        return True

Since I did not want to touch the s, I declared new_s to store alpha numeric characters. Here are steps I went through to solve the problem.

1. Check each character whether it is alpha numeric with isalnum() method. If True, add the character to new_s in lower case

2. Check if the string is empty

3. Check characters in new_s from the start and from the end at the same time. The checking will continue until it reaches the middle with range(len(new_s)//2). If the length is odd, the rest after dividing the length of the sentence by 2 will be discarded, since it is unnecessary to consider the character right in the middle.

4. If the current position(left side) as new_s[i] and the other current position(right side) new_s[-(i+1)] are not equal, it means no palindrome thus return False, otherwise return True

Reflection

Palindrome, to me, is always something that excites me, perhaps all problems related to string control do excite me.

I at first was solving the problem by checking alpha numeric characters in ascii value comparison like, if (c.lower() >= "a" and c.lower() <= "z") or (c.lower() >= "0" and c.lower() <= "9"):. I am not sure if it is common to compare and check the characters in this way, but I did it anyways. But then I found out that there is a method isalnum() as an internal python library and wrote this code down. I thought this would give me some time efficiency when using the method but it was nothing of a difference. It just makes the code look simpler.