Valid Palindrome Two Pointer in DSA Python - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the main idea behind the two-pointer technique in checking a palindrome?

Use two pointers starting at the beginning and end of the string, moving towards the center while comparing characters to check if they match.

beginner

Why do we skip non-alphanumeric characters in the Valid Palindrome problem?

Because the problem asks to ignore spaces, punctuation, and symbols, focusing only on letters and numbers for palindrome checking.

beginner

In the two-pointer palindrome check, what happens if characters at the pointers don't match?

We conclude the string is not a palindrome and stop the check early.

beginner

How do you handle case sensitivity in the Valid Palindrome problem using two pointers?

Convert characters to the same case (usually lowercase) before comparing to ignore case differences.

intermediate

What is the time complexity of the two-pointer approach for checking a palindrome?

O(n), where n is the length of the string, because each character is checked at most once.

What do the two pointers represent in the Valid Palindrome two-pointer approach?

A Start and end indices of the string

B Middle characters of the string

C Random positions in the string

D Only the first half of the string

The two pointers start at the beginning and end of the string and move towards the center.

Which characters are ignored when checking for a valid palindrome?

A Only letters

B Non-alphanumeric characters like spaces and punctuation

C Only numbers

D All characters are considered

Non-alphanumeric characters are skipped to focus on letters and numbers only.

If characters at the two pointers do not match, what should you do?

A Return false immediately

B Continue checking

C Skip both characters

D Reverse the string

Mismatch means the string is not a palindrome, so return false immediately.

How do you handle uppercase and lowercase letters in the palindrome check?

A Compare as is

B Convert both to uppercase before comparing

C Ignore letters

D Convert both to lowercase before comparing

Converting both characters to lowercase ensures case-insensitive comparison.

What is the best description of the time complexity of the two-pointer palindrome check?

A O(n^2)

B O(n log n)

C O(n)

D O(1)

Each character is checked once, so the time complexity is linear, O(n).

Explain how the two-pointer technique works to check if a string is a valid palindrome.

Think about how you compare characters from both ends moving towards the center.

Describe why ignoring non-alphanumeric characters and case differences is important in the Valid Palindrome problem.

Consider how sentences with spaces and punctuation can still be palindromes.

Practice

1. What does the two-pointer technique check when verifying if a string is a palindrome?

easy

A. It counts the number of vowels in the string

B. It compares characters from the start and end moving towards the center

C. It reverses the string and compares with the original

D. It sorts the characters and checks if sorted string equals original

Solution

1. Step 1: Understand two-pointer technique

   Two pointers start at the beginning and end of the string.

2. Step 2: Compare characters moving inward

   Characters at both pointers are compared; pointers move closer if equal.

3. Final Answer:

   It compares characters from the start and end moving towards the center -> Option B

4. Quick Check:

   Two-pointer palindrome check = comparing ends [OK]

Hint: Compare characters from both ends moving inward [OK]

Common Mistakes:

• Thinking it reverses the string
• Checking only vowels or sorting characters
• Using one pointer only

2. Which of the following is the correct syntax to initialize two pointers for a palindrome check in Python?

easy

A. left = 1; right = len(s)

B. left = -1; right = len(s) + 1

C. left = len(s); right = 0

D. left = 0; right = len(s) - 1

Solution

1. Step 1: Identify pointer start positions

   Left pointer starts at 0 (first character), right pointer at last index (length - 1).

2. Step 2: Check syntax correctness

   left = 0; right = len(s) - 1 correctly sets left=0 and right=len(s)-1 for valid indexing.

3. Final Answer:

   left = 0; right = len(s) - 1 -> Option D

4. Quick Check:

   Start pointers at 0 and last index [OK]

Hint: Start left at 0 and right at length minus one [OK]

Common Mistakes:

• Starting right pointer at length (out of range)
• Starting left pointer at 1 (skips first char)
• Using negative or invalid indices

3. What will be the output of the following code?

def is_palindrome(s):
    left, right = 0, len(s) - 1
    while left < right:
        if s[left] != s[right]:
            return False
        left += 1
        right -= 1
    return True

print(is_palindrome('racecar'))
print(is_palindrome('hello'))

medium

A. True\nTrue

B. False\nTrue

C. True\nFalse

D. False\nFalse

Solution

1. Step 1: Check 'racecar' palindrome

   Characters match from both ends, so returns True.

2. Step 2: Check 'hello' palindrome

   Characters differ ('h' != 'o'), returns False.

3. Final Answer:

   True\nFalse -> Option C

4. Quick Check:

   'racecar' palindrome = True, 'hello' not = False [OK]

Hint: Check characters from ends; mismatch means False [OK]

Common Mistakes:

• Assuming 'hello' is palindrome
• Returning True without full check
• Mixing up True/False outputs

4. Find the error in this palindrome check code:

def check_palindrome(s):
    left, right = 0, len(s)
    while left < right:
        if s[left] != s[right]:
            return False
        left += 1
        right -= 1
    return True

medium

A. right should be initialized as len(s) - 1

B. left should start at 1

C. The while condition should be left <= right

D. The function should return False at the end

Solution

1. Step 1: Check pointer initialization

   Right pointer is set to len(s), which is out of string index range.

2. Step 2: Correct pointer initialization

   Right should be len(s) - 1 to point to last character index.

3. Final Answer:

   right should be initialized as len(s) - 1 -> Option A

4. Quick Check:

   Right pointer must be last index, not length [OK]

Hint: Right pointer = length minus one, not length [OK]

Common Mistakes:

• Using len(s) as index causes IndexError
• Changing left pointer start incorrectly
• Incorrect loop condition

5. Given the string s = "A man, a plan, a canal: Panama", which modification is needed in the two-pointer palindrome check to correctly identify it as a palindrome?

hard

A. Ignore non-alphanumeric characters and case during comparison

B. Reverse the string and compare directly

C. Only compare vowels in the string

D. Convert string to uppercase and compare characters directly

Solution

1. Step 1: Understand input complexity

   String contains spaces, commas, and mixed case which affect direct comparison.

2. Step 2: Modify comparison logic

   Skip non-alphanumeric characters and compare lowercase versions to ignore case and symbols.

3. Step 3: Why other options fail

   Reversing or uppercase alone won't ignore spaces/punctuation; vowels only is incorrect.

4. Final Answer:

   Ignore non-alphanumeric characters and case during comparison -> Option A

5. Quick Check:

   Clean string and ignore case for palindrome check [OK]

Hint: Skip symbols and compare lowercase chars [OK]

Common Mistakes:

• Not ignoring punctuation and spaces
• Only changing case without skipping symbols
• Comparing vowels only