Valid Palindrome Two Pointer in DSA Python - Build from Scratch

Check if a String is a Palindrome Using Two Pointers

📖 Scenario: Imagine you are building a simple text checker that tells if a word or phrase reads the same forwards and backwards, ignoring spaces, punctuation, and letter case. This is called a palindrome.For example, "Race car" is a palindrome because if you ignore spaces and case, it reads the same backwards.

🎯 Goal: You will write a program that uses two pointers to check if a given string is a palindrome. The program will ignore spaces, punctuation, and case differences.

📋 What You'll Learn

Create a string variable with a specific sentence.

Create two pointer variables to track positions in the string.

Use a loop to move the pointers and compare characters.

Print True if the string is a palindrome, otherwise False.

💡 Why This Matters

🌍 Real World

Palindrome checks are used in text processing, data validation, and coding puzzles.

💼 Career

Understanding two-pointer techniques helps in solving string and array problems efficiently in software development.

1

Create the string to check

Create a string variable called text and set it to the exact value "A man, a plan, a canal: Panama".

DSA Python

# Create the string variable called text
# Your code here

Hint

Use quotes to create the string exactly as shown.

2

Set up two pointers

Create two integer variables called left and right. Set left to 0 and right to len(text) - 1.

DSA Python

text = "A man, a plan, a canal: Panama"
# Create left and right pointers
# Your code here

Hint

Use len(text) to find the length of the string.

3

Use two pointers to check palindrome

Write a while loop that runs while left < right. Inside the loop, skip non-alphanumeric characters by moving left forward and right backward. Compare the lowercase characters at left and right. If they differ, set a variable is_palindrome to False and break the loop. If they match, move left forward and right backward. Before the loop, set is_palindrome to True.

DSA Python

text = "A man, a plan, a canal: Panama"
left = 0
right = len(text) - 1

# Use a while loop to check palindrome with two pointers
# Your code here

Hint

Use isalnum() to check if a character is a letter or number.

Use lower() to ignore case differences.

4

Print the result

Write a print statement to display the value of is_palindrome.

DSA Python

text = "A man, a plan, a canal: Panama"
left = 0
right = len(text) - 1
is_palindrome = True

while left < right:
    while left < right and not text[left].isalnum():
        left += 1
    while left < right and not text[right].isalnum():
        right -= 1
    if text[left].lower() != text[right].lower():
        is_palindrome = False
        break
    left += 1
    right -= 1

# Print the result
# Your code here

Hint

The output should be True because the text is a palindrome.