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.