Valid Palindrome Two Pointer in DSA C# - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the main idea behind the two-pointer technique in checking a palindrome?

Use two pointers starting at the beginning and end of the string, move them towards the center, and compare characters to check if they match.

beginner

Why do we skip non-alphanumeric characters in the Valid Palindrome problem?

Because only letters and numbers matter for palindrome checking; spaces, punctuation, and symbols are ignored.

beginner

In the two-pointer palindrome check, what happens if characters at pointers do not match?

We conclude the string is not a palindrome and stop checking further.

beginner

How do you handle case differences when checking for palindrome?

Convert both characters to the same case (usually lowercase) before comparing.

intermediate

What is the time complexity of the two-pointer palindrome check?

O(n), where n is the length of the string, because each character is checked at most once.

What do the two pointers represent in the Valid Palindrome Two Pointer approach?

A Start and end indices of the string

B Middle characters of the string

C Random positions in the string

D Only the first half of the string

The two pointers start at the beginning and end of the string and move towards the center.

Which characters are ignored when checking for a valid palindrome?

A Alphanumeric characters

B Letters only

C Spaces and punctuation

D Numbers only

Spaces, punctuation, and other non-alphanumeric characters are skipped.

How do you compare characters in a case-insensitive palindrome check?

A Convert both to uppercase

B Convert both to lowercase

C Compare as is

D Ignore case differences

Converting both characters to lowercase ensures case-insensitive comparison.

What should you do if characters at the two pointers do not match?

A Move pointers inward and continue

B Skip one pointer

C Restart checking from the beginning

D Return false immediately

If characters don't match, the string is not a palindrome, so return false.

What is the overall time complexity of the two-pointer palindrome check?

A O(n)

B O(n log n)

C O(1)

D O(n^2)

Each character is checked once, so the time complexity is linear, O(n).

Explain how the two-pointer technique works to check if a string is a valid palindrome.

Think about comparing characters from both ends towards the center.

Describe how to handle characters that are not letters or numbers when checking for a palindrome.

Focus on ignoring spaces and punctuation.

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