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

Bs.size() - 1

C0

D-1

Attempts:

3 left

💡 Hint

Common Mistakes

Starting left pointer at 1 skips the first character.

Using s.size() - 1 sets pointer at the end, not start.

The left pointer should start at index 0 to begin checking from the start 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'

A-1

Bs.size()

C0

Ds.size() - 1

Attempts:

3 left

💡 Hint

Common Mistakes

Using s.size() causes out-of-bounds error.

Starting at 0 sets pointer at the start, not the end.

The right pointer should start at the last character index, which is size minus one.

3fill in blank

hard

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

DSA C++

while ([1] < right) {

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

Aleft

Bs.size()

Cright

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'right < right' makes the condition always false, preventing the loop from executing.

Using 's.size() < right' is incorrect comparison.

The loop should run while the left pointer is less than the right pointer.

4fill in blank

hard

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

DSA C++

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

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

Aleft

Bright

C0

Ds.size() - 1

Attempts:

3 left

💡 Hint

Common Mistakes

Using fixed indices like 0 or s.size() - 1 causes wrong skipping.

Mixing left and right pointers in conditions.

We check characters at left and right pointers to skip non-alphanumeric characters.

5fill in blank

hard

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

DSA C++

if (tolower(s[[1]]) != tolower(s[[2]])) return false;
left[3];
right--;

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

Aleft

Bright

C++

D--

Attempts:

3 left

💡 Hint

Common Mistakes

Using '--' on left pointer moves it backward incorrectly.

Comparing characters without tolower causes case mismatch.

Compare characters at left and right pointers ignoring case, then move left pointer forward and right pointer 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 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