Longest Palindromic Substring in DSA C# - Build from Scratch

Longest Palindromic Substring

📖 Scenario: You are working on a text editor feature that highlights the longest palindrome in a given text. A palindrome is a word or phrase that reads the same backward as forward, like "madam" or "racecar".

🎯 Goal: Build a program that finds the longest palindromic substring inside a given string.

📋 What You'll Learn

Create a string variable with a specific value.

Create helper variables to track the longest palindrome.

Implement a method to expand around the center to find palindromes.

Use a loop to check all possible centers for palindromes.

Print the longest palindromic substring found.

💡 Why This Matters

🌍 Real World

Finding palindromes is useful in text analysis, DNA sequence analysis, and error detection in data.

💼 Career

Understanding string manipulation and substring search algorithms is important for software developers working with text processing and data analysis.

1

Create the input string

Create a string variable called input and set it to the value "babad".

DSA C#

# Create a string variable called input and set it to "babad"
# Your code here

Hint

Use string input = "babad"; to create the variable.

2

Create variables to track the longest palindrome

Create two integer variables called start and maxLength. Set start to 0 and maxLength to 1.

DSA C#

string input = "babad";
// Create two integer variables start and maxLength
// Your code here

Hint

Use int start = 0; and int maxLength = 1;.

3

Implement the palindrome expansion method

Create a method called ExpandAroundCenter that takes three parameters: string s, int left, and int right. It should return the length of the palindrome by expanding around the center indices left and right.

DSA C#

string input = "babad";
int start = 0;
int maxLength = 1;

// Create method ExpandAroundCenter
// Your code here

Hint

Use a while loop to expand as long as characters match and indices are valid.

4

Find and print the longest palindromic substring

Use a for loop with variable i from 0 to input.Length - 1. For each i, find the length of odd and even palindromes by calling ExpandAroundCenter(input, i, i) and ExpandAroundCenter(input, i, i + 1). Update start and maxLength if a longer palindrome is found. Finally, print the longest palindromic substring using Console.WriteLine.

DSA C#

string input = "babad";
int start = 0;
int maxLength = 1;

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

// Use a for loop to find the longest palindrome and print it
// Your code here

Hint

Check both odd and even length palindromes around each center.

Use input.Substring(start, maxLength) to get the longest palindrome.

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