Longest Palindromic Substring in DSA C++ - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to initialize the maximum length of palindrome found so far.

DSA C++

int maxLength = [1];

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

A-1

B1

C0

Dstring::npos

Attempts:

3 left

💡 Hint

Common Mistakes

Initializing maxLength to 1 before checking any substring.

We start with maxLength = 0 because initially no palindrome substring is found.

2fill in blank

medium

Complete the code to expand around the center and return the length of palindrome.

DSA C++

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

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

A2

B1

C0

D-1

Attempts:

3 left

💡 Hint

Common Mistakes

Returning right - left without subtracting 1.

Returning right - left - 2.

We subtract 1 because after the loop exits, 'right' points one past the end and 'left' points one before the start of the palindrome, so right - left - 1 gives the correct length.

3fill in blank

hard

Fix the error in the code to update the start index when a longer palindrome is found.

DSA C++

if (len > maxLength) {
    maxLength = len;
    start = i - [1] / 2;
}

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

Alen - 1

Blen / 2

Clen + 1

Dlen

Attempts:

3 left

💡 Hint

Common Mistakes

Using len directly without subtracting 1.

We subtract 1 from len before dividing by 2 to correctly find the start index for even length palindromes.

4fill in blank

hard

Fill both blanks to correctly call the expand function for odd and even length palindromes.

DSA C++

int len1 = expandAroundCenter(s, i, i);
int len2 = expandAroundCenter(s, i, [1]);
int len = std::max(len1, [2]);

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

Ai + 1

Bi - 1

Clen1

Dlen2

Attempts:

3 left

💡 Hint

Common Mistakes

Using i - 1 instead of i + 1 for even length.

For even length palindrome, we expand around centers i and i+1. Then take max of len1 and len2.

5fill in blank

hard

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

DSA C++

std::string longestPalindrome(std::string s) {
    int start = 0, maxLength = 1;
    for (int i = 0; i < s.size(); i++) {
        int len1 = expandAroundCenter(s, i, i);
        int len2 = expandAroundCenter(s, i, i + 1);
        int len = std::max(len1, len2);
        if (len > maxLength) {
            maxLength = len;
            start = i - [1] / 2;
        }
    }
    return s.substr(start, [2]);
}

int expandAroundCenter(const std::string& s, int left, int right) {
    while (left >= 0 && right < s.size() && s[left] == s[right]) {
        left--;
        right++;
    }
    return right - left - [3];
}

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

Alen - 1

BmaxLength

C2

D1

Attempts:

3 left

💡 Hint

Common Mistakes

Using incorrect divisor or substring length.

Start index is adjusted by (len - 1)/2. The substring length is maxLength. The expand function subtracts 1 after expansion.

Practice

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

easy

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

B. Find the longest substring with all unique characters

C. Find the longest substring with increasing characters

D. Find the longest substring that contains only vowels

Solution

1. Step 1: Understand the problem statement

   The problem asks to find the longest part of a string that is a palindrome, meaning it reads the same forwards and backwards.

2. Step 2: Compare options with the problem goal

   Only Find the longest substring that reads the same forwards and backwards describes finding a substring that reads the same forwards and backwards, which matches the palindrome definition.

3. Final Answer:

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

4. Quick Check:

   Longest Palindromic Substring = Find the longest substring that reads the same forwards and backwards [OK]

Hint: Look for substring that reads same forwards and backwards [OK]

Common Mistakes:

• Confusing palindrome with unique characters
• Thinking about substrings with vowels only
• Mixing substring with subsequence

2. Which of the following C++ code snippets correctly initializes a 2D vector dp of size n x n with false values for the Longest Palindromic Substring problem?

easy

A. std::vector dp(n, std::vector<int>(n, false));

B. std::vector<bool> dp(n, std::vector<bool>(n, false));

C. std::vector dp(n, n, false);

D. std::vector dp(n, std::vector<bool>(n, false));

Solution

1. Step 1: Understand 2D vector initialization syntax

   To create a 2D vector of booleans with size n x n, use vector of vectors: std::vector dp(n, std::vector<bool>(n, false));

2. Step 2: Check each option's correctness

   std::vector dp(n, std::vector<bool>(n, false)); uses correct syntax. std::vector<bool> dp(n, std::vector<bool>(n, false)); tries to assign vector<bool> to vector<bool>, which is invalid. std::vector dp(n, std::vector<int>(n, false)); uses int instead of bool. std::vector dp(n, n, false); uses invalid constructor syntax.

3. Final Answer:

   std::vector dp(n, std::vector<bool>(n, false)); -> Option D

4. Quick Check:

   Correct 2D vector initialization = std::vector dp(n, std::vector<bool>(n, false)); [OK]

Hint: Use vector of vectors with inner vector initialized [OK]

Common Mistakes:

• Using wrong data type for dp vector
• Incorrect constructor syntax for 2D vector
• Confusing vector<bool> with vector

3. What is the output of the following C++ code snippet that finds the longest palindromic substring length?

std::string s = "babad";
int n = s.size();
int maxLength = 1;
for (int i = 0; i < n; i++) {
  for (int j = i; j < n; j++) {
    int left = i, right = j;
    bool isPalindrome = true;
    while (left < right) {
      if (s[left] != s[right]) {
        isPalindrome = false;
        break;
      }
      left++; right--;
    }
    if (isPalindrome && (j - i + 1) > maxLength) {
      maxLength = j - i + 1;
    }
  }
}
std::cout << maxLength << std::endl;

medium

A. 3

B. 2

C. 4

D. 5

Solution

1. Step 1: Understand the code logic

   The code checks all substrings of "babad" and updates maxLength if the substring is a palindrome and longer than current maxLength.

2. Step 2: Identify longest palindromic substrings in "babad"

   "bab" and "aba" are palindromes of length 3. No longer palindrome exists.

3. Final Answer:

   3 -> Option A

4. Quick Check:

   Longest palindrome length in "babad" = 3 [OK]

Hint: Check all substrings and update max length if palindrome [OK]

Common Mistakes:

• Assuming entire string is palindrome
• Counting non-palindromic substrings
• Off-by-one errors in substring length

4. Identify the bug in the following C++ code snippet intended to find the longest palindromic substring length:

std::string s = "cbbd";
int n = s.size();
int maxLength = 1;
for (int i = 0; i < n; i++) {
  for (int j = i; j < n; j++) {
    int left = i, right = j;
    while (left < right) {
      if (s[left] != s[right]) {
        break;
      }
      left++; right--;
    }
    if ((j - i + 1) > maxLength) {
      maxLength = j - i + 1;
    }
  }
}
std::cout << maxLength << std::endl;

medium

A. Incorrect loop ranges for i and j

B. Wrong initialization of maxLength

C. Missing check if substring is palindrome before updating maxLength

D. Using break instead of continue in while loop

Solution

1. Step 1: Analyze palindrome check logic

   The code breaks out of while loop if characters don't match but does not track if substring is palindrome.

2. Step 2: Check maxLength update condition

   maxLength is updated without confirming substring is palindrome, causing incorrect results.

3. Final Answer:

   Missing check if substring is palindrome before updating maxLength -> Option C

4. Quick Check:

   Must confirm palindrome before updating maxLength [OK]

Hint: Always confirm palindrome before updating max length [OK]

Common Mistakes:

• Updating maxLength without palindrome check
• Confusing break and continue in loops
• Incorrect loop boundaries

5. Given the string "forgeeksskeegfor", which approach below efficiently finds the longest palindromic substring and what is its length?

hard

A. Dynamic Programming approach, length 7

B. Expand Around Center approach, length 10

C. Brute Force approach, length 12

D. Manacher's Algorithm, length 8

Solution

1. Step 1: Identify longest palindrome in "forgeeksskeegfor"

   The longest palindromic substring is "geeksskeeg" with length 10.

2. Step 2: Choose efficient approach

   Expand Around Center approach efficiently finds palindromes by expanding from each center, suitable for this problem.

3. Final Answer:

   Expand Around Center approach, length 10 -> Option B

4. Quick Check:

   Longest palindrome length = 10 using Expand Around Center [OK]

Hint: Expand from centers to find longest palindrome efficiently [OK]

Common Mistakes:

• Confusing length of palindrome substring
• Choosing less efficient brute force for large strings
• Mixing up algorithm names and their complexities