Longest Palindromic Substring in DSA Python - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to initialize the longest palindrome substring as empty.

DSA Python

longest = [1]

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

A0

BNone

C""

D[]

Attempts:

3 left

💡 Hint

Common Mistakes

Using None instead of an empty string.

Using a list or number instead of a string.

We start with an empty string to store the longest palindrome found.

2fill in blank

medium

Complete the code to expand around the center and return the palindrome substring.

DSA Python

def expand_around_center(s, left, right):
    while left >= 0 and right < len(s) and s[left] == s[right]:
        left -= 1
        right += 1
    return s[[1]:right]

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

Aleft - 1

Bright - 1

Cleft

Dleft + 1

Attempts:

3 left

💡 Hint

Common Mistakes

Returning s[left:right] which misses the first character.

Returning s[left-1:right] which is out of bounds.

After the loop, left and right are one step beyond the palindrome bounds, so we return s[left+1:right].

3fill in blank

hard

Fix the error in the code to update the longest palindrome substring correctly.

DSA Python

if len(current) > len(longest):
    longest = [1]

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

As

Bcurrent

Clongest

Dleft

Attempts:

3 left

💡 Hint

Common Mistakes

Assigning longest to the whole string s.

Assigning longest to left index instead of substring.

We update longest to the current palindrome substring if it is longer.

4fill in blank

hard

Fill both blanks to correctly find odd length palindromes by expanding around center i.

DSA Python

for i in range(len(s)):
    current = expand_around_center(s, [1], [2])

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

Ai

Bi + 1

Ci - 1

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Using different indices for left and right causing wrong palindrome detection.

Starting from 0 instead of current index i.

For odd length palindromes, expand around the same center index i.

5fill in blank

hard

Fill all three blanks to find even length palindromes by expanding around centers i and i+1.

DSA Python

for i in range(len(s)):
    current = expand_around_center(s, [1], [2])
    if len(current) > len(longest):
        longest = [3]

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

Ai

Bi + 1

Ccurrent

Dlongest

Attempts:

3 left

💡 Hint

Common Mistakes

Using same index for left and right causing odd length palindrome only.

Not updating longest with current substring.

For even length palindromes, expand around i and i+1, then update longest if current is longer.