Longest Palindromic Substring in DSA C# - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to return the longest palindromic substring length.

DSA C#

public int LongestPalindromeLength(string s) {
    int maxLength = 1;
    for (int i = 0; i < s.Length; i++) {
        int len1 = ExpandAroundCenter(s, i, i);
        int len2 = ExpandAroundCenter(s, i, i + [1]);
        maxLength = Math.Max(maxLength, Math.Max(len1, len2));
    }
    return maxLength;
}

private int ExpandAroundCenter(string s, int left, int right) {
    while (left >= 0 && right < s.Length && 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

Using 0 instead of 1 causes checking the same center twice.

Using 2 or -1 causes index errors or wrong palindrome checks.

The second center for even-length palindromes starts at i and i+1, so the offset is 1.

2fill in blank

medium

Complete the code to update the start index of the longest palindrome found.

DSA C#

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

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

Alen - 1

Blen

Clen + 1

Dlen * 2

Attempts:

3 left

💡 Hint

Common Mistakes

Using len instead of len - 1 causes off-by-one errors.

Using len + 1 or len * 2 causes incorrect start positions.

To find the start index, subtract half of the palindrome length minus one from the center index.

3fill in blank

hard

Fix the error in the palindrome expansion loop condition.

DSA C#

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

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

Aright - 1

Bright

Cleft + 1

Dleft

Attempts:

3 left

💡 Hint

Common Mistakes

Using right for both indices causes comparing the same character.

Using left + 1 or right - 1 causes index errors.

We compare characters at left and right indices, so s[left] must be compared to s[right].

4fill in blank

hard

Fill both blanks to correctly extract the longest palindromic substring.

DSA C#

return s.Substring([1], [2]);

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

Astart

BmaxLength

Ci

Dlen

Attempts:

3 left

💡 Hint

Common Mistakes

Using i instead of start causes wrong substring extraction.

Using len instead of maxLength causes inconsistent length.

Substring needs the start index and the length of the palindrome.

5fill in blank

hard

Fill all three blanks to complete the palindrome length calculation and update logic.

DSA C#

int len1 = ExpandAroundCenter(s, i, i);
int len2 = ExpandAroundCenter(s, i, i + [1]);
int len = Math.Max(len1, len2);
if (len > maxLength) {
    maxLength = len;
    start = i - (len - 1) / [2];
}

return s.Substring(start, [3]);

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

A1

B2

CmaxLength

Dlen

Attempts:

3 left

💡 Hint

Common Mistakes

Using 0 instead of 1 for the second center causes wrong palindrome checks.

Using len instead of maxLength for substring length causes errors.

The second center offset is 1; dividing by 2 finds half length; substring length is maxLength.

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. Count all palindromic substrings in the string

C. Reverse the entire string

D. Find the longest substring without repeating characters

Solution

1. Step 1: Understand the problem definition

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

2. Step 2: Differentiate from similar problems

   Counting all palindromic substrings or reversing the string are different tasks. The focus here is on the longest palindromic substring only.

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 mirrors itself exactly [OK]

Common Mistakes:

• Confusing substring with subsequence
• Counting all palindromes instead of longest
• Trying to reverse entire string

2. Which of the following C# code snippets correctly initializes a 2D boolean array dp for dynamic programming in the Longest Palindromic Substring problem?

easy

A. bool dp = new bool[s.Length, s.Length];

B. bool[][] dp = new bool[s.Length][];

C. bool[,] dp = new bool[s.Length, s.Length];

D. bool dp[][] = new bool[s.Length][s.Length];

Solution

1. Step 1: Understand C# array syntax

   In C#, a 2D array is declared as type[,] name = new type[size1, size2];. bool[,] dp = new bool[s.Length, s.Length]; uses this correct syntax.

2. Step 2: Identify incorrect syntax

   bool[][] dp = new bool[s.Length][]; declares jagged arrays but misses initializing inner arrays. bool dp[][] = new bool[s.Length][s.Length]; uses invalid syntax for C#. bool dp = new bool[s.Length, s.Length]; tries to declare a single boolean variable with array sizes, which is invalid.

3. Final Answer:

   bool[,] dp = new bool[s.Length, s.Length]; -> Option C

4. Quick Check:

   2D array in C# = bool[,] dp = new bool[s.Length, s.Length]; [OK]

Hint: Use comma inside brackets for 2D arrays in C# [OK]

Common Mistakes:

• Using jagged array syntax without initializing inner arrays
• Confusing C# array syntax with other languages
• Declaring arrays without proper dimensions

3. What will be the output of the following C# code snippet that finds the longest palindromic substring?

string s = "babad";
string result = LongestPalindrome(s);
Console.WriteLine(result);

string LongestPalindrome(string str) {
    int start = 0, maxLength = 1;
    int len = str.Length;
    bool[,] dp = new bool[len, len];
    for (int i = 0; i < len; i++) dp[i, i] = true;
    for (int length = 2; length <= len; length++) {
        for (int i = 0; i <= len - length; i++) {
            int j = i + length - 1;
            if (str[i] == str[j]) {
                if (length == 2 || dp[i + 1, j - 1]) {
                    dp[i, j] = true;
                    if (length > maxLength) {
                        start = i;
                        maxLength = length;
                    }
                }
            }
        }
    }
    return str.Substring(start, maxLength);
}

medium

A. "aba"

B. "bab"

C. "bad"

D. "babad"

Solution

1. Step 1: Understand the input and algorithm

   The input string is "babad". The code uses dynamic programming to find the longest palindromic substring by checking substrings of increasing length.

2. Step 2: Trace the longest palindrome found

   Both "bab" and "aba" are palindromes of length 3. The code updates start and maxLength when it finds a longer palindrome. It finds "bab" first and returns it.

3. Final Answer:

   "bab" -> Option B

4. Quick Check:

   Longest palindrome in "babad" = "bab" [OK]

Hint: DP finds first longest palindrome substring [OK]

Common Mistakes:

• Assuming "aba" is returned instead of "bab"
• Returning entire string instead of substring
• Ignoring substring length updates

4. Identify the bug in the following C# code snippet for finding the longest palindromic substring:

string LongestPalindrome(string s) {
    int n = s.Length;
    bool[,] dp = new bool[n, n];
    int maxLen = 1, start = 0;
    for (int i = 0; i < n; i++) dp[i, i] = true;
    for (int length = 2; length <= n; length++) {
        for (int i = 0; i < n - length; i++) {
            int j = i + length - 1;
            if (s[i] == s[j]) {
                if (length == 2 || dp[i + 1, j - 1]) {
                    dp[i, j] = true;
                    if (length > maxLen) {
                        start = i;
                        maxLen = length;
                    }
                }
            }
        }
    }
    return s.Substring(start, maxLen);
}

medium

A. The inner loop condition should be < i < n - length + 1

B. The substring method parameters are incorrect

C. The dp array is not initialized for single characters

D. The code does not check for palindrome substrings of length 1

Solution

1. Step 1: Analyze the inner loop boundary

   The inner loop runs with condition i < n - length. For substring length 'length', the last valid start index is n - length, but the loop only goes up to n - length - 1, missing it. The loop condition should be i <= n - length or equivalently i < n - length + 1 to include the last substring.

2. Step 2: Identify off-by-one error

   This is an off-by-one error in the inner loop condition. Other options are incorrect because the dp array is initialized for single characters, the substring parameters are correct, and length 1 substrings are handled.

3. Final Answer:

   The inner loop condition should be < i < n - length + 1 -> Option A

4. Quick Check:

   Loop boundary off-by-one = The inner loop condition should be < i < n - length + 1 [OK]

Hint: Check loop boundaries carefully for substring indices [OK]

Common Mistakes:

• Ignoring off-by-one errors in loops
• Misunderstanding substring parameters
• Assuming dp initialization is missing

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

hard

A. Expand Around Center approach; substring is "forgeeksskeegfor"

B. Dynamic Programming approach; substring is "forgeeksskeegfor"

C. Brute Force approach; substring is "forgeeksskeegfor"

D. Expand Around Center approach; substring is "geeksskeeg"

Solution

1. Step 1: Understand the input string and palindrome

   The string "forgeeksskeegfor" contains the palindrome "geeksskeeg" in the middle, which is the longest palindromic substring.

2. Step 2: Identify correct approach and substring

   The Expand Around Center approach efficiently finds palindromes by expanding from each center. It will find "geeksskeeg" correctly. The entire string is not a palindrome, so options B, C, and D are incorrect.

3. Final Answer:

   Expand Around Center approach; substring is "geeksskeeg" -> Option D

4. Quick Check:

   Longest palindrome = "geeksskeeg" via center expansion [OK]

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

Common Mistakes:

• Assuming entire string is palindrome
• Confusing brute force with efficient methods
• Missing the correct longest substring