Palindrome Detection in DSA C# - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to check if a string is a palindrome by reversing it.

DSA C#

string reversed = new string(input.Reverse().ToArray());
bool isPalindrome = input [1] reversed;

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

A==

B!=

C>

D<

Attempts:

3 left

💡 Hint

Common Mistakes

Using '!=' instead of '==' causes wrong palindrome detection.

Using comparison operators like '>' or '<' is incorrect for string equality.

We compare the original string with its reversed version using '=='. If they are equal, the string is a palindrome.

2fill in blank

medium

Complete the code to convert the input string to lowercase before checking palindrome.

DSA C#

string lowerInput = input.[1]();
string reversed = new string(lowerInput.Reverse().ToArray());
bool isPalindrome = lowerInput == reversed;

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

AToLower

BToUpper

CTrim

DReplace

Attempts:

3 left

💡 Hint

Common Mistakes

Using ToUpper instead of ToLower changes the case but is inconsistent with the reversed string.

Using Trim or Replace does not change letter case.

Converting the string to lowercase ensures the palindrome check is case-insensitive.

3fill in blank

hard

Fix the error in the loop that compares characters from start and end to detect palindrome.

DSA C#

for (int i = 0; i < input.Length / 2; i++) {
    if (input[i] [1] input[input.Length - 1 - i]) {
        return false;
    }
}
return true;

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

A==

B<

C>

D!=

Attempts:

3 left

💡 Hint

Common Mistakes

Using '==' causes the function to return false for palindromes.

Using comparison operators like '>' or '<' is incorrect for character equality.

We return false if characters at mirrored positions are not equal, so the condition uses '!='.

4fill in blank

hard

Fill both blanks to create a dictionary comprehension that maps words to their palindrome status.

DSA C#

var palindromeStatus = words.ToDictionary(word => word, word => word.[1]() == new string(word.[2]().Reverse().ToArray()));

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

AToLower

BToUpper

Attempts:

3 left

💡 Hint

Common Mistakes

Using different case conversions on each side causes incorrect results.

Using ToUpper instead of ToLower is acceptable but inconsistent here.

Converting words to lowercase before comparison ensures case-insensitive palindrome detection.

5fill in blank

hard

Fill all three blanks to filter a list of words and create a dictionary of palindromes only.

DSA C#

var palindromes = words.Where(word => word.[1]() == new string(word.[2]().Reverse().ToArray()))
    .ToDictionary([3] => [3], [3] => true);

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

AToLower

Bword

Dw

Attempts:

3 left

💡 Hint

Common Mistakes

Using different variable names inconsistently causes errors.

Not converting to lowercase causes case-sensitive mismatches.

Convert words to lowercase for comparison and use a variable name 'w' for dictionary keys and values.

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