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Grokking the Coding Interview Patterns in Python

Valid Palindrome

Explore how to efficiently validate if a string is a palindrome using the two pointers technique. Learn to ignore non-alphanumeric characters and case differences while solving this common coding interview problem. This lesson helps you break down the problem and implement a clear solution in a hands-on coding environment.

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Statement

Given a string, s, return TRUE if it is a palindrome; otherwise, return FALSE.

A phrase is considered a palindrome if it reads the same backward as forward after converting all uppercase letters to lowercase and removing any characters that are not letters or numbers. Only alphanumeric characters (letters and digits) are taken into account.

Constraints:

  • 1≤1 \leq s.length ≤3000\leq 3000

  • s consists only of printable ASCII characters.

Examples

canvasAnimation-image
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Understand the problem

Let’s take a moment to make sure you've correctly understood the problem. The quiz below helps you check if you're solving the correct problem:

Valid Palindrome

1.

What is the output for the following input?

s = “A man, a plan, a canal: Panama”

A.

TRUE

B.

FALSE

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Figure it out!

We have a game for you to play. Rearrange the logical building blocks to develop a clearer understanding on how to solve this problem.

Sequence - Vertical
Drag and drop the cards to rearrange them in the correct sequence.
1
2
3
4
5

Try it yourself

Implement your solution in the following coding playground.

Need a nudge?

Explore these hints—each one is designed to guide you a step closer to the solution.

Python
usercode > Solution.py
def is_palindrome(s):
  left, right = 0, len(s) - 1
  while left < right:
      while left < right and not s[left].isalnum():
        left += 1
      while left < right and not s[right].isalnum():
        right -= 1
      if s[left].lower() != s[right].lower():
        # Replace this placeholder "pass" statement with your code
        pass
      
      left += 1
      right -= 1
  return True
Valid Palindrome