Valid Palindrome Two Pointer in DSA C# - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to initialize the left pointer at the start of the string.

DSA C#

int left = [1];

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

A1

B0

Cs.Length

D-1

Attempts:

3 left

💡 Hint

Common Mistakes

Starting left pointer at 1 skips the first character.

Using s.Length causes out of range errors.

The left pointer should start at index 0, the beginning of the string.

2fill in blank

medium

Complete the code to initialize the right pointer at the end of the string.

DSA C#

int right = [1];

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

As.Length - 1

B0

Cs.Length

D-1

Attempts:

3 left

💡 Hint

Common Mistakes

Using s.Length causes index out of range errors.

Starting at 0 means left and right pointers start at the same place.

The right pointer should start at the last character, which is at index s.Length - 1.

3fill in blank

hard

Fix the error in the while loop condition to continue while left is less than right.

DSA C#

while (left [1] right) {

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

A>

B>=

C<

D<=

Attempts:

3 left

💡 Hint

Common Mistakes

Using > causes the loop to never run.

Using <= causes extra unnecessary comparisons.

The loop should run while left is less than right to compare characters from both ends.

4fill in blank

hard

Fill both blanks to skip non-alphanumeric characters from the left and right pointers.

DSA C#

while (left < right && !char.IsLetterOrDigit(s[1])) left++;
while (left < right && !char.IsLetterOrDigit(s[2])) right--;

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

A[left]

B[right]

C.ToLower()

D.Trim()

Attempts:

3 left

💡 Hint

Common Mistakes

Using .ToLower() or .Trim() here is incorrect because we need to access characters by index.

Missing the index access causes compile errors.

We check characters at left and right indexes using s[left] and s[right] to skip non-alphanumeric characters.

5fill in blank

hard

Fill all four blanks to compare characters ignoring case and move pointers accordingly.

DSA C#

if (char.ToLower(s[1]) != char.ToLower(s[2]))
    return false;
left[3];
right[4];

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

A[left]

B[right]

C++

D--

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong indexes or forgetting to move pointers causes infinite loops.

Not ignoring case causes false negatives.

We compare characters at left and right indexes ignoring case, then move left forward and right backward.

Practice

1. What is the main idea behind the two-pointer technique to check if a string is a palindrome?

easy

A. Sort the characters and check if the sorted string equals the original

B. Reverse the string and compare it with the original string

C. Count the frequency of each character and check if all counts are even

D. Use two pointers starting from both ends and move towards the center comparing characters

Solution

1. Step 1: Understand the two-pointer approach

   Two pointers start at the beginning and end of the string and move inward.

2. Step 2: Compare characters at pointers

   If characters are equal, move pointers closer; if not, string is not palindrome.

3. Final Answer:

   Use two pointers starting from both ends and move towards the center comparing characters -> Option D

4. Quick Check:

   Two-pointer technique = Compare ends inward [OK]

Hint: Compare characters from both ends moving inward [OK]

Common Mistakes:

• Reversing string instead of using pointers
• Checking only half the string incorrectly
• Ignoring case or non-alphanumeric characters

2. Which of the following C# code snippets correctly initializes two pointers for palindrome checking on string s?

easy

A. int left = 0, right = s.Length - 1;

B. int left = 1, right = s.Length;

C. int left = 0, right = s.Length;

D. int left = 1, right = s.Length - 1;

Solution

1. Step 1: Identify valid pointer initialization

   Left pointer starts at 0 (first char), right pointer at s.Length - 1 (last char).

2. Step 2: Check each option

   int left = 0, right = s.Length - 1; correctly sets left=0 and right=s.Length-1; others are off by one.

3. Final Answer:

   int left = 0, right = s.Length - 1; -> Option A

4. Quick Check:

   Pointers start at 0 and last index [OK]

Hint: Start left at 0 and right at length minus one [OK]

Common Mistakes:

• Setting right pointer to s.Length (out of range)
• Starting left pointer at 1 instead of 0
• Confusing length with last index

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

string s = "A man, a plan, a canal: Panama";
bool IsPalindrome(string str) {
    int left = 0, right = str.Length - 1;
    while (left < right) {
        if (!char.IsLetterOrDigit(str[left])) { left++; continue; }
        if (!char.IsLetterOrDigit(str[right])) { right--; continue; }
        if (char.ToLower(str[left]) != char.ToLower(str[right])) return false;
        left++; right--;
    }
    return true;
}
Console.WriteLine(IsPalindrome(s));

medium

A. Runtime exception

B. True

C. Compilation error

D. False

Solution

1. Step 1: Understand the code logic

   The code skips non-alphanumeric characters and compares letters ignoring case.

2. Step 2: Analyze the input string

   "A man, a plan, a canal: Panama" is a famous palindrome ignoring spaces and punctuation.

3. Final Answer:

   True -> Option B

4. Quick Check:

   Ignoring punctuation and case, string is palindrome [OK]

Hint: Ignore non-letters and compare lowercase chars [OK]

Common Mistakes:

• Not skipping non-alphanumeric characters
• Comparing characters without case normalization
• Assuming spaces affect palindrome check

4. Identify the bug in the following palindrome check code and select the fix:

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;
}

medium

A. Change while condition to left < right

B. Skip non-alphanumeric characters before comparing

C. All of the above

D. Add case-insensitive comparison using char.ToLower

Solution

1. Step 1: Check loop condition correctness

   Using <= causes unnecessary comparison when left == right; < is better.

2. Step 2: Consider case and character filtering

   Comparisons should ignore case and skip non-alphanumeric chars for valid palindrome check.

3. Step 3: Combine fixes

   All suggested fixes improve correctness and handle common palindrome cases.

4. Final Answer:

   All of the above -> Option C

5. Quick Check:

   Loop, case, and filtering fixes needed [OK]

Hint: Check loop, case, and skip non-letters [OK]

Common Mistakes:

• Using <= instead of < in loop
• Ignoring case differences
• Not skipping punctuation or spaces

5. Given the string "No lemon, no melon", which modification to the two-pointer palindrome check is necessary to correctly identify it as a palindrome?

hard

A. Convert characters to lowercase and skip non-alphanumeric characters during comparison

B. Only convert characters to lowercase without skipping any characters

C. Skip spaces only, but compare punctuation as is

D. Reverse the string and compare directly without any preprocessing

Solution

1. Step 1: Analyze the string content

   "No lemon, no melon" contains spaces and punctuation that should be ignored.

2. Step 2: Determine necessary preprocessing

   Lowercase conversion and skipping all non-alphanumeric characters ensures correct palindrome check.

3. Final Answer:

   Convert characters to lowercase and skip non-alphanumeric characters during comparison -> Option A

4. Quick Check:

   Lowercase + skip non-alphanumeric = correct palindrome check [OK]

Hint: Lowercase and skip all non-letters/digits [OK]

Common Mistakes:

• Skipping only spaces but not punctuation
• Ignoring case differences
• Comparing raw strings without preprocessing