Longest Palindromic Substring in DSA Typescript - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to initialize the longest palindrome substring as the first character.

DSA Typescript

let longest = s[1](0, 1);

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

Asubstring

BcharAt

Cslice

Dsubstr

Attempts:

3 left

💡 Hint

Common Mistakes

Using charAt returns a single character, not a substring.

Using slice also works but substring is preferred here.

substring(0, 1) extracts the first character as a string, which is the initial longest palindrome.

2fill in blank

medium

Complete the code to expand around the center indices left and right.

DSA Typescript

while (left >= 0 && right < s.length && s[1](left) === s[1](right)) { left--; right++; }

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

Asubstring

BcharAt

Cslice

Dsubstr

Attempts:

3 left

💡 Hint

Common Mistakes

Using substring or slice returns substrings, not single characters, causing wrong comparisons.

charAt(index) returns the character at the given index, which is needed for comparison.

3fill in blank

hard

Fix the error in updating the longest palindrome substring after expanding.

DSA Typescript

if (right - left - 1 > longest.length) {
  longest = s[1](left + 1, right);
}

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

Aslice

Bsubstr

Csubstring

DcharAt

Attempts:

3 left

💡 Hint

Common Mistakes

Using charAt returns a single character, not a substring.

Using substr has different parameters and is less preferred.

substring(left + 1, right) correctly extracts the palindrome substring after expansion.

4fill in blank

hard

Fill both blanks to define the helper function that expands around center indices.

DSA Typescript

function expandAroundCenter(s: string, [1]: number, [2]: number): string {
  let left = [1], right = [2];
  while (left >= 0 && right < s.length && s.charAt(left) === s.charAt(right)) {
    left--;
    right++;
  }
  return s.substring(left + 1, right);
}

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

Aleft

Bright

Cstart

Dend

Attempts:

3 left

💡 Hint

Common Mistakes

Using start and end as parameter names is less clear here.

Mixing parameter names with variable names inside the function.

The function parameters are left and right indices to expand around.

5fill in blank

hard

Fill all three blanks to complete the main function that finds the longest palindromic substring.

DSA Typescript

function longestPalindrome(s: string): string {
  if (s.length < 1) return "";
  let longest = s.substring(0, 1);
  for (let i = 0; i < s.length; i++) {
    const odd = expandAroundCenter(s, i, [1]);
    const even = expandAroundCenter(s, i, [2]);
    const longer = odd.length > even.length ? odd : even;
    if (longer.length > longest.length) {
      longest = [3];
    }
  }
  return longest;
}

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

Ai

Bi + 1

Clonger

Ds

Attempts:

3 left

💡 Hint

Common Mistakes

Using i-1 or i+2 for centers causes wrong expansions.

Assigning longest to s instead of the longer palindrome substring.

For odd length palindrome, expand around (i, i). For even length, expand around (i, i+1). Update longest with the longer palindrome.

Practice

1. What is the main goal of the Longest Palindromic Substring problem?

easy

A. Find the longest substring with all unique characters

B. Find the longest substring that reads the same forwards and backwards

C. Find the longest substring with increasing characters

D. Find the longest substring that contains only vowels

Solution

1. Step 1: Understand palindrome definition

   A palindrome reads the same forwards and backwards, like 'madam' or 'racecar'.

2. Step 2: Identify the problem goal

   The problem asks to find the longest substring inside a string that is a palindrome.

3. Final Answer:

   Find the longest substring that reads the same forwards and backwards -> Option B

4. Quick Check:

   Palindrome substring = longest palindrome [OK]

Hint: Look for substrings that mirror around a center [OK]

Common Mistakes:

• Confusing palindrome with unique characters
• Looking for longest substring without repeats
• Thinking about substrings with vowels only

2. Which of the following TypeScript function signatures correctly represents a function to find the longest palindromic substring?

easy

A. function longestPalindrome(s: string): string

B. function longestPalindrome(s: number): number

C. function longestPalindrome(s: string): number[]

D. function longestPalindrome(s: boolean): string

Solution

1. Step 1: Check input parameter type

   The input is a string, so parameter type must be string.

2. Step 2: Check return type

   The function returns the longest palindromic substring, which is a string.

3. Final Answer:

   function longestPalindrome(s: string): string -> Option A

4. Quick Check:

   Input string, output string [OK]

Hint: Input and output are both strings for this problem [OK]

Common Mistakes:

• Using wrong input type like number or boolean
• Returning array instead of string
• Mixing input and output types

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

function longestPalindrome(s: string): string {
  let start = 0, maxLength = 1;
  for (let i = 0; i < s.length; i++) {
    let low = i, high = i;
    while (low >= 0 && high < s.length && s[low] === s[high]) {
      if (high - low + 1 > maxLength) {
        start = low;
        maxLength = high - low + 1;
      }
      low--;
      high++;
    }
  }
  return s.substring(start, start + maxLength);
}

console.log(longestPalindrome('babad'))

medium

A. "bab"

B. "aba"

C. "bad"

D. "babad"

Solution

1. Step 1: Understand the code logic

   The code expands around each character to find odd-length palindromes and tracks the longest one.

2. Step 2: Trace the input 'babad'

   Expanding around index 1 ('a') finds 'bab' (length 3). Around index 2 ('b') finds 'aba' (length 3). Both have length 3, but 'bab' is found first and stored.

3. Final Answer:

   "bab" -> Option A

4. Quick Check:

   First longest palindrome found = "bab" [OK]

Hint: Expand around center and pick first longest palindrome [OK]

Common Mistakes:

• Assuming 'aba' is always returned instead of 'bab'
• Returning entire string incorrectly
• Confusing substring indices

4. Identify the bug in the following TypeScript code for finding the longest palindromic substring:

function longestPalindrome(s: string): string {
  let start = 0, maxLength = 1;
  for (let i = 0; i < s.length; i++) {
    let low = i, high = i + 1;
    while (low >= 0 && high < s.length && s[low] === s[high]) {
      if (high - low + 1 > maxLength) {
        start = low;
        maxLength = high - low + 1;
      }
      low--;
      high++;
    }
  }
  return s.substring(start, start + maxLength);
}

medium

A. The loop should start from 1 instead of 0

B. The substring indices are incorrect in the return statement

C. The code only checks even-length palindromes, missing odd-length ones

D. The variables low and high are initialized incorrectly for odd-length palindromes

Solution

1. Step 1: Analyze initialization of low and high

   low = i and high = i + 1 means it only checks even-length palindromes centered between i and i+1.

2. Step 2: Identify missing odd-length palindrome check

   Odd-length palindromes centered at i (low = high = i) are not checked, so some palindromes are missed.

3. Final Answer:

   The code only checks even-length palindromes, missing odd-length ones -> Option C

4. Quick Check:

   Only even-length palindrome check = bug [OK]

Hint: Check both odd (low=high=i) and even (low=i, high=i+1) centers [OK]

Common Mistakes:

• Checking only even-length palindromes
• Not expanding around single character centers
• Incorrect loop start index

5. Given the string "forgeeksskeegfor", which approach correctly finds the longest palindromic substring and what is that substring?

hard

A. Manacher's algorithm; longest palindrome is "forgeeksskeegfor"

B. Dynamic programming approach; longest palindrome is "forgeeksskeegfor"

C. Brute force checking all substrings; longest palindrome is "ss"

D. Expand around center approach; longest palindrome is "geeksskeeg"

Solution

1. Step 1: Identify the longest palindrome in the string

   The substring "geeksskeeg" is the longest palindrome inside "forgeeksskeegfor".

2. Step 2: Match approach with correct palindrome

   Expand around center approach finds "geeksskeeg" correctly. The entire string is not a palindrome, so options A and B are incorrect. Brute force (C) incorrectly identifies "ss" as the longest.

3. Final Answer:

   Expand around center approach; longest palindrome is "geeksskeeg" -> Option D

4. Quick Check:

   Center expansion finds "geeksskeeg" [OK]

Hint: Use center expansion for efficient palindrome search [OK]

Common Mistakes:

• Assuming entire string is palindrome
• Confusing brute force with efficient methods
• Mixing up palindrome substrings