Valid Palindrome Two Pointer in DSA Python - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to initialize the left pointer at the start of the string.

DSA Python

left = [1]

Drag options to blanks, or click blank then click option'

A0

B1

Clen(s)

D-1

Attempts:

3 left

💡 Hint

Common Mistakes

Starting left pointer at 1 or -1 causes skipping or errors.

The left pointer should start at index 0 to begin checking from the start of the string.

2fill in blank

medium

Complete the code to initialize the right pointer at the end of the string.

DSA Python

right = [1]

Drag options to blanks, or click blank then click option'

Alen(s)

Blen(s) - 1

C0

D-1

Attempts:

3 left

💡 Hint

Common Mistakes

Using len(s) causes index out of range errors.

The right pointer should start at the last character index, which is length minus one.

3fill in blank

hard

Fix the error in the while loop condition to continue while left is less than right.

DSA Python

while [1] < right:

Drag options to blanks, or click blank then click option'

Aright - 1

Bleft + 1

Cleft

Dright

Attempts:

3 left

💡 Hint

Common Mistakes

Using right or expressions causes wrong loop behavior.

The loop should run while the left pointer is less than the right pointer to compare characters.

4fill in blank

hard

Fill both blanks to skip non-alphanumeric characters from left and right pointers.

DSA Python

while left < right and not s[left].[1]():
    left += 1
while left < right and not s[right].[2]():
    right -= 1

Drag options to blanks, or click blank then click option'

Aisalnum

Bisalpha

Cisdigit

Disspace

Attempts:

3 left

💡 Hint

Common Mistakes

Using isalpha() skips digits incorrectly.

Using isspace() skips letters incorrectly.

isalnum() checks if a character is a letter or number, so we skip characters that are not alphanumeric.

5fill in blank

hard

Fill all three blanks to compare characters ignoring case and move pointers inward.

DSA Python

if s[left].[1]() != s[right].[2]():
    return False
left += [3]
right -= 1

Drag options to blanks, or click blank then click option'

Alower

Bupper

C1

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Not converting case causes wrong comparisons.

Moving pointers incorrectly causes infinite loops.

We convert both characters to lowercase to compare ignoring case, then move left pointer by 1 and right pointer by 1.

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