Valid Palindrome Two Pointer in DSA C++ - Build from Scratch

Valid Palindrome Two Pointer

📖 Scenario: You are building a simple text checker that tells if a word or phrase reads the same forwards and backwards, ignoring spaces and punctuation. This is called a palindrome.For example, "racecar" is a palindrome, but "hello" is not.

🎯 Goal: Build a program that uses two pointers to check if a given string is a palindrome, ignoring non-letter characters and case differences.

📋 What You'll Learn

Create a string variable called text with the exact value "A man, a plan, a canal: Panama".

Create two integer variables called left and right to point to the start and end of text.

Use a while loop to move left and right pointers towards the center, skipping non-alphabetic characters.

Compare characters at left and right ignoring case to check palindrome condition.

Print "Palindrome" if the string is a palindrome, otherwise print "Not Palindrome".

💡 Why This Matters

🌍 Real World

Checking palindromes is useful in text processing, data validation, and coding interviews.

💼 Career

Understanding two-pointer techniques helps in solving many string and array problems efficiently in software development.

1

Create the input string

Create a string variable called text and set it to the exact value "A man, a plan, a canal: Panama".

DSA C++

# Create the string variable text with the given value
# Your code here

Hint

Use std::string to create the variable text and assign the exact phrase.

2

Set up two pointers

Create two integer variables called left and right. Set left to 0 and right to text.size() - 1.

DSA C++

std::string text = "A man, a plan, a canal: Panama";
// Create left and right pointers
# Your code here

Hint

Use int to create left and right and initialize them to the start and end indices of text.

3

Implement the two-pointer palindrome check

Write a while loop that runs while left is less than right. Inside the loop, skip non-alphabetic characters by moving left forward and right backward. Compare characters at left and right ignoring case. If they differ, break the loop.

DSA C++

std::string text = "A man, a plan, a canal: Panama";
int left = 0;
int right = text.size() - 1;
// Use a while loop to check palindrome with two pointers
# Your code here

Hint

Use isalpha() to check letters and tolower() to compare characters ignoring case.

4

Print the result

After the loop, print "Palindrome" if left is greater than or equal to right, otherwise print "Not Palindrome".

DSA C++

std::string text = "A man, a plan, a canal: Panama";
int left = 0;
int right = text.size() - 1;

while (left < right) {
    while (left < right && !isalpha(text[left])) {
        left++;
    }
    while (left < right && !isalpha(text[right])) {
        right--;
    }
    if (tolower(text[left]) != tolower(text[right])) {
        break;
    }
    left++;
    right--;
}

// Print the result
# Your code here

Hint

Use std::cout to print the final result based on the pointers.

Practice

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

easy

A. Sort the string and check if it matches the original

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

C. Reverse the string and compare with the original

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

Solution

1. Step 1: Understand 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 differ, string is not palindrome; if all match, it is palindrome.

3. Final Answer:

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

4. Quick Check:

   Two-pointer technique = Use two pointers starting from both ends and move towards the center comparing characters [OK]

Hint: Compare characters from ends moving inward [OK]

Common Mistakes:

• Trying to sort string instead of comparing characters
• Only checking half the string without pointers
• 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.size() - 1;

B. int left = 1, right = s.size();

C. int left = s.size(), right = 0;

D. int left = s.size() - 1, right = 0;

Solution

1. Step 1: Identify valid pointer initialization

   Left pointer starts at 0 (first char), right pointer at last index (size - 1).

2. Step 2: Check each option

   int left = 0, right = s.size() - 1; correctly sets left=0 and right=s.size()-1; others have invalid indices.

3. Final Answer:

   int left = 0, right = s.size() - 1; -> Option A

4. Quick Check:

   Pointers start at 0 and size-1 = int left = 0, right = s.size() - 1; [OK]

Hint: Left = 0, Right = length - 1 for string indices [OK]

Common Mistakes:

• Starting right pointer at s.size() (out of range)
• Swapping left and right initialization
• Starting left pointer at 1 instead of 0

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

std::string s = "A man, a plan, a canal: Panama";
int left = 0, right = s.size() - 1;
while (left < right) {
  while (left < right && !std::isalnum(s[left])) left++;
  while (left < right && !std::isalnum(s[right])) right--;
  if (std::tolower(s[left]) != std::tolower(s[right])) {
    std::cout << "Not Palindrome" << std::endl;
    return 0;
  }
  left++;
  right--;
}
std::cout << "Palindrome" << std::endl;

medium

A. Compilation Error

B. Not Palindrome

C. Runtime Error

D. Palindrome

Solution

1. Step 1: Understand the code logic

   It skips non-alphanumeric chars and compares characters case-insensitively from both ends.

2. Step 2: Check if string is palindrome ignoring cases and symbols

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

3. Final Answer:

   Palindrome -> Option D

4. Quick Check:

   Ignoring non-alnum and case, string is palindrome = Palindrome [OK]

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

Common Mistakes:

• Not skipping non-alphanumeric characters
• Comparing characters case-sensitively
• Confusing output with runtime or compile errors

4. Identify the bug in this palindrome check code snippet:

std::string s = "racecar";
int left = 0, right = s.size();
while (left < right) {
  if (s[left] != s[right]) {
    std::cout << "Not Palindrome" << std::endl;
    break;
  }
  left++;
  right--;
}
std::cout << "Palindrome" << std::endl;

medium

A. Right pointer initialized to s.size() which is out of range

B. Left pointer should start at 1

C. Missing tolower() for case-insensitive check

D. Loop condition should be left <= right

Solution

1. Step 1: Check pointer initialization

   Right pointer is set to s.size(), which is one past the last valid index.

2. Step 2: Understand consequences

   Accessing s[right] causes out-of-bounds error or undefined behavior.

3. Final Answer:

   Right pointer initialized to s.size() which is out of range -> Option A

4. Quick Check:

   Right pointer must be s.size()-1, not s.size() [OK]

Hint: Right pointer = length - 1 to avoid out-of-range [OK]

Common Mistakes:

• Using s.size() as last index
• Ignoring out-of-bounds access errors
• Assuming loop condition fixes index error

5. Given a string s, how would you modify the two-pointer palindrome check to ignore spaces, punctuation, and be case-insensitive? Choose the best approach.

hard

A. Convert string to uppercase and compare characters including spaces

B. Reverse the string and compare directly without any changes

C. Use two pointers; skip non-alphanumeric chars; compare lowercase characters

D. Count characters frequency and check if all are even

Solution

1. Step 1: Handle non-alphanumeric characters

   Skip spaces and punctuation by moving pointers until alphanumeric chars found.

2. Step 2: Compare characters case-insensitively

   Convert characters to lowercase before comparing to ignore case differences.

3. Final Answer:

   Use two pointers; skip non-alphanumeric chars; compare lowercase characters -> Option C

4. Quick Check:

   Skip non-alnum + lowercase compare = Use two pointers; skip non-alphanumeric chars; compare lowercase characters [OK]

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

Common Mistakes:

• Comparing characters without skipping punctuation
• Ignoring case differences
• Using frequency count which doesn't check order