Palindrome Detection in DSA C# - Practice Problems & Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of Palindrome Check for a Simple String

What is the output of the following C# code that checks if a string is a palindrome?

DSA C#

using System;

class Program {
    static bool IsPalindrome(string s) {
        int left = 0, right = s.Length - 1;
        while (left < right) {
            if (s[left] != s[right]) return false;
            left++;
            right--;
        }
        return true;
    }

    static void Main() {
        string test = "radar";
        Console.WriteLine(IsPalindrome(test));
    }
}

A True

B False

C Compilation error

D Runtime exception

💡 Hint

Check if the string reads the same forwards and backwards.

The string "radar" is the same when reversed, so the function returns true.

❓ Predict Output

intermediate

Output of Palindrome Check with Mixed Case and Spaces

What is the output of this C# code that checks if a string is a palindrome ignoring case and spaces?

DSA C#

using System;
using System.Text.RegularExpressions;

class Program {
    static bool IsPalindrome(string s) {
        s = Regex.Replace(s.ToLower(), "[^a-z0-9]", "");
        int left = 0, right = s.Length - 1;
        while (left < right) {
            if (s[left] != s[right]) return false;
            left++;
            right--;
        }
        return true;
    }

    static void Main() {
        string test = "A man a plan a canal Panama";
        Console.WriteLine(IsPalindrome(test));
    }
}

A Compilation error due to Regex

B False

C True

D Runtime exception

💡 Hint

Ignore spaces and case when checking palindrome.

After removing non-alphanumeric characters and ignoring case, the string reads the same forwards and backwards.

🔧 Debug

advanced

Identify the Error in Palindrome Function

What error will this C# code produce when checking if a string is a palindrome?

DSA C#

using System;

class Program {
    static bool IsPalindrome(string s) {
        int left = 0, right = s.Length;
        while (left < right) {
            if (s[left] != s[right]) return false;
            left++;
            right--;
        }
        return true;
    }

    static void Main() {
        string test = "level";
        Console.WriteLine(IsPalindrome(test));
    }
}

A IndexOutOfRangeException

B Returns false

C Returns true

D Compilation error

💡 Hint

Check the initial value of the right pointer.

The right pointer is set to s.Length which is out of bounds for indexing, causing an exception.

❓ Predict Output

advanced

Output of Palindrome Check Using Recursion

What is the output of this recursive C# palindrome check for the input "deified"?

DSA C#

using System;

class Program {
    static bool IsPalindrome(string s, int left, int right) {
        if (left >= right) return true;
        if (s[left] != s[right]) return false;
        return IsPalindrome(s, left + 1, right - 1);
    }

    static void Main() {
        string test = "deified";
        Console.WriteLine(IsPalindrome(test, 0, test.Length - 1));
    }
}

A StackOverflowException

B False

C Compilation error

D True

💡 Hint

Recursion checks characters from outside to inside.

The string "deified" is a palindrome, so the recursive function returns true.

🧠 Conceptual

expert

Minimum Number of Changes to Make a String Palindrome

Given a string, what is the minimum number of character changes needed to make it a palindrome? For example, for "abcd", the answer is 2 because changing 'c' to 'b' and 'd' to 'a' makes "abba" which is a palindrome. Which approach correctly computes this?

A Sort the string and count characters that appear an odd number of times.

B Use two pointers from start and end, increment count when characters differ, then move pointers inward.

C Reverse the string and count all positions where characters differ.

D Count all unique characters and divide by two.

💡 Hint

Focus on pairs of characters from both ends.

Only pairs of mismatched characters need one change each to match, so counting mismatches with two pointers gives the minimum changes.

Practice

1. What is a palindrome in the context of strings?

easy

A. A string that contains only vowels

B. A string that reads the same forwards and backwards

C. A string with all unique characters

D. A string that is empty

Solution

1. Step 1: Understand palindrome definition

   A palindrome is a word or phrase that reads the same forwards and backwards, ignoring spaces and punctuation.

2. Step 2: Apply definition to options

   A string that reads the same forwards and backwards matches this definition exactly, while others describe different string properties.

3. Final Answer:

   A string that reads the same forwards and backwards -> Option B

4. Quick Check:

   Palindrome = Same forwards and backwards [OK]

Hint: Check if string reversed equals original [OK]

Common Mistakes:

• Confusing palindrome with unique characters
• Thinking palindrome means only vowels
• Assuming empty string is always palindrome

2. Which of the following C# code snippets correctly checks if a string s is a palindrome?

easy

A. return s.Length == 0;

B. return s == s.ToUpper();

C. return s == new string(s.Reverse().ToArray());

D. return s[0] == s[s.Length];

Solution

1. Step 1: Understand palindrome check logic

   To check palindrome, compare string with its reversed version.

2. Step 2: Analyze each option

   return s == new string(s.Reverse().ToArray()); reverses string and compares correctly. return s == s.ToUpper(); compares uppercase, not palindrome. return s.Length == 0; checks empty string only. return s[0] == s[s.Length]; accesses invalid index (out of range).

3. Final Answer:

   return s == new string(s.Reverse().ToArray()); -> Option C

4. Quick Check:

   Reverse and compare = return s == new string(s.Reverse().ToArray()); [OK]

Hint: Reverse string and compare to original [OK]

Common Mistakes:

• Using wrong index causing out of range error
• Comparing uppercase instead of reversed string
• Checking only empty string

3. What will be the output of the following C# code?

string s = "level";
bool isPalindrome = s == new string(s.Reverse().ToArray());
Console.WriteLine(isPalindrome);

medium

A. True

B. False

C. Compilation error

D. Runtime exception

Solution

1. Step 1: Reverse the string "level"

   Reversed string is "level", same as original.

2. Step 2: Compare original and reversed strings

   Since both are equal, isPalindrome is true, so Console.WriteLine prints True.

3. Final Answer:

   True -> Option A

4. Quick Check:

   "level" reversed = "level" [OK]

Hint: Check if reversed string equals original [OK]

Common Mistakes:

• Assuming output is False for palindromes
• Confusing compilation error due to Reverse()
• Thinking runtime exception occurs

4. Identify the error in this palindrome check code and select the fix:

string s = "test";
bool isPalindrome = true;
for (int i = 0; i <= s.Length / 2; i++)
{
    if (s[i] != s[s.Length - i])
    {
        isPalindrome = false;
        break;
    }
}
Console.WriteLine(isPalindrome);

medium

A. Remove the break statement

B. Change loop condition to i < s.Length

C. Initialize isPalindrome to false

D. Change s.Length - i to s.Length - i - 1

Solution

1. Step 1: Identify index error in character comparison

   String indices go from 0 to s.Length - 1, so s[s.Length - i] is out of range when i=0.

2. Step 2: Fix index to s.Length - i - 1

   This correctly accesses the mirrored character from the end.

3. Final Answer:

   Change s.Length - i to s.Length - i - 1 -> Option D

4. Quick Check:

   Correct index fixes out of range error [OK]

Hint: Remember last index is length minus one [OK]

Common Mistakes:

• Using s.Length - i instead of s.Length - i - 1
• Looping beyond half the string length
• Removing break causing unnecessary checks

5. You want to detect if a string is a palindrome ignoring case and spaces. Which code snippet correctly implements this in C#?

hard

A. string clean = new string(s.Where(char.IsLetterOrDigit).ToArray()).ToLower();
bool isPal = clean == new string(clean.Reverse().ToArray());

B. string clean = s.Replace(" ", "");
bool isPal = clean == clean.ToUpper();

C. bool isPal = s == new string(s.Reverse().ToArray());

D. bool isPal = s.ToLower() == s.ToUpper();

Solution

1. Step 1: Clean string by removing non-letter/digit and convert to lowercase

   string clean = new string(s.Where(char.IsLetterOrDigit).ToArray()).ToLower(); bool isPal = clean == new string(clean.Reverse().ToArray()); uses Where with char.IsLetterOrDigit and ToLower to normalize string.

2. Step 2: Reverse cleaned string and compare

   string clean = new string(s.Where(char.IsLetterOrDigit).ToArray()).ToLower(); bool isPal = clean == new string(clean.Reverse().ToArray()); reverses cleaned string and compares correctly for palindrome ignoring case and spaces.

3. Final Answer:

   string clean = new string(s.Where(char.IsLetterOrDigit).ToArray()).ToLower(); bool isPal = clean == new string(clean.Reverse().ToArray()); -> Option A

4. Quick Check:

   Normalize then reverse and compare = string clean = new string(s.Where(char.IsLetterOrDigit).ToArray()).ToLower(); bool isPal = clean == new string(clean.Reverse().ToArray()); [OK]

Hint: Remove spaces and lowercase before checking palindrome [OK]

Common Mistakes:

• Not removing spaces or punctuation
• Comparing original string without normalization
• Using uppercase comparison incorrectly