patternpythonMinor

Valid palindrome solution

Submitted by: @import:stackexchange-codereview·Mar 10, 2026·

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palindromesolutionvalid

Problem

I'm working on this valid palindrome problem. Any advice on code bug, better idea for low algorithm execution time complexity, code style, etc. are highly appreciated.

Problem

Given a string, determine if it is a palindrome, considering only
alphanumeric characters and ignoring cases.

Example

"A man, a plan, a canal: Panama" is a palindrome. "race a car" is not
a palindrome.

Note

Have you consider that the string might be empty? This is a good
question to ask during an interview. For the purpose of this problem,
we define empty string as valid palindrome.

Source Code

def check_valid(source):
    i = 0
    j = len(source)-1
    source =source.lower()
    while i  j:
            return True

if __name__ == "__main__":
    print check_valid('A man, a plan, a canal: Panama') # return True
    print check_valid('race a car') # return False

Solution

Some small nitpicks:

Instead of i

Here: ('a'

It seems like you always forget to put a space between operators: i+=1 should be i += 1.

Add docstrings to your function

Reviewed code:

def check_valid(source):
    """ Return True/False if a string is a palindrome """

    i, j, source = 0, len(source) - 1, source.lower()
    while i  j:
            return True

if __name__ == "__main__":
    print check_valid('A man, a plan, a canal: Panama')  # return True
    print check_valid('race a car')  # return False

Code Snippets

def check_valid(source):
    """ Return True/False if a string is a palindrome """

    i, j, source = 0, len(source) - 1, source.lower()
    while i <= j:
        while i < j and not source[i].isalnum():
            i += 1
        while i < j and not source[j].isalnum():
            j -= 1
        if i <= j:
            if source[i] != source[j]:
                return False
            else:
                i += 1
                j -= 1
        if i > j:
            return True


if __name__ == "__main__":
    print check_valid('A man, a plan, a canal: Panama')  # return True
    print check_valid('race a car')  # return False

Context

StackExchange Code Review Q#149835, answer score: 4

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