String Processing in Python: Is Palindrome

In this video, we will be considering how to test whether a string is a palindrome in Python. We will be doing this using a linear amount of time and a constant amount of space.

We will cover how to solve this problem algorithmically, and then code up a solution to this question in Python.

This video is part of a series on string processing and, specifically, on how these problems tend to show up in the context of a technical interview:

This video is also part of an "Algorithm" series. For more algorithm tutorials:

The software written in this video is available at:

Do you like the development environment I'm using in this video? It's a customized version of vim that's enhanced for Python development. If you want to see how I set up my vim, I have a series on this here:

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[AAAAAAAAAAAAAAAAAAAAHH]

I don't think the while loops in the while loop are necessary, they can just be conditionals, with the i<j condition removed

[Alan-qbqt]

Wonderful tutorial. This solved my problems on palindrome.

[sankaranarayanansubbayya]

s = ''.join([i for i in s if i.isalpha()]).lower()
will work to clean up the string. .replace(' ', '') is not needed. .isalpha() filters out the space.

[mightyprogrammer]

i was little bit confused from last 2 hours

but now i have got what am was understand in this solution

My doubt was
why we take j = len(s) -1 instead of j = len(s)

but afeter so many searching i got the answer, that in a string index number starts from 0

anyway your explanation is so well
you are the best for DSA

[zvapa]

Thanks for the videos! Very nice examples and clear explanations. Keep up the good work! Subscribed. 👍

[redpred]

For the solution using the join() method. I don't think the argument needs to be within a list. Also, isn't the replace() method pointless as white-space isn't alphabetical?

[elisad]

Super helpful video. I was wondering why you added the` i < j` condition in the 2 inner while loops (one for s[i] and one for s[j]) that checks if the char is alphanumeric? Since the outer loop already accounts for `i < j` condition... is there a corner case I'm missing? Thank you!

[shivamrawat]

sir i am little bit confuse hope you will get me out of it....why we first increment the j and then decrement it later

[connorhennen]

Is stack a better data structure for this problem?

[chittybabu]

should the j be decremented inside the while loop ? (increment will shift to right and not to left) ....I saw your edit at the end of the you

[McMurphyMillions]

couldn't you just use
txt = input("enter a palindrome: ")
txt = ''.join([i for i in txt if i.isalpha()]).replace(" ", "").lower()

if txt == txt[::-1]:
print(True)

[nwaiwumunachimso]

Hello. thanks for the video. Can you explain the time complexity for your second solution? My guess is O(n^2) but I am not sure since my initial loop only goes half the time

[rajivm]

Aren’t you looping over the string several times, by which you are increasing the time complexity?

[Kingofqueers]

the variable s isn't a palindrome, cat spelled backwards is tac..