Valid Palindrome Two Pointer in DSA Rust - 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 don't 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 technique?

A Random positions in the string

B Middle characters of the string

C Only the first character

D Start and end indices of the string

Two pointers start at the beginning and end of the string to compare characters moving inward.

Which characters are ignored when checking for a valid palindrome?

A Spaces and punctuation

B Alphanumeric characters

C Letters only

D Numbers only

Spaces, punctuation, and symbols are ignored; only letters and numbers are considered.

If characters at two pointers differ, what should the algorithm do?

A Continue checking

B Return false immediately

C Skip both characters

D Convert characters to uppercase

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

How do you make the palindrome check case-insensitive?

A Only check lowercase letters

B Only check uppercase letters

C Ignore case by converting both characters to lowercase

D Compare ASCII values directly

Convert both characters to lowercase before comparing to ignore case differences.

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

A O(n)

B O(1)

C O(log n)

D O(n^2)

Each character is checked at most 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 moving towards the center.

Describe why ignoring non-alphanumeric characters is important in the Valid Palindrome problem.

Consider how sentences with spaces and punctuation can still be palindromes.

Practice

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

easy

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

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

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

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

Solution

1. Step 1: Understand two-pointer approach

   The two-pointer technique uses one pointer at the start and one at the end of the string.

2. Step 2: Compare characters moving inward

   Both pointers move towards the center, comparing characters at each step to check equality.

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 both ends moving inward [OK]

Common Mistakes:

• Thinking reversing string is the two-pointer method
• Checking character counts instead of positions
• Sorting string changes order, not palindrome check

2. Which Rust code snippet correctly initializes two pointers for palindrome checking on a string s?

easy

A. let (mut left, mut right) = (0, s.len() - 1);

B. let (left, right) = (0, s.len());

C. let (mut left, mut right) = (1, s.len() - 1);

D. let (left, right) = (1, s.len());

Solution

1. Step 1: Identify pointer initialization for indices

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

2. Step 2: Check mutability and correct indices

   Both pointers must be mutable to move; right pointer must be s.len() - 1 to avoid out-of-bounds.

3. Final Answer:

   let (mut left, mut right) = (0, s.len() - 1); -> Option A

4. Quick Check:

   Correct pointer init = let (mut left, mut right) = (0, s.len() - 1); [OK]

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

Common Mistakes:

• Using s.len() as right index causes out-of-bounds
• Not making pointers mutable
• Starting left pointer at 1 skips first character

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

fn is_palindrome(s: &str) -> bool {
    let bytes = s.as_bytes();
    let (mut left, mut right) = (0, bytes.len() - 1);
    while left < right {
        if bytes[left] != bytes[right] {
            return false;
        }
        left += 1;
        right -= 1;
    }
    true
}

fn main() {
    println!("{}", is_palindrome("racecar"));
    println!("{}", is_palindrome("hello"));
}

medium

A. false\nfalse

B. false\ntrue

C. true\nfalse

D. true\ntrue

Solution

1. Step 1: Check palindrome for "racecar"

   Characters match from both ends moving inward, so returns true.

2. Step 2: Check palindrome for "hello"

   Characters differ at first comparison ('h' vs 'o'), so returns false.

3. Final Answer:

   true\nfalse -> Option C

4. Quick Check:

   racecar = true, hello = false [OK]

Hint: Check first and last chars; mismatch means false [OK]

Common Mistakes:

• Assuming hello is palindrome
• Confusing output order
• Ignoring early return on mismatch

4. Identify the bug in this Rust function that checks palindrome and select the fix:

fn is_palindrome(s: &str) -> bool {
    let bytes = s.as_bytes();
    let (mut left, mut right) = (0, bytes.len());
    while left < right {
        if bytes[left] != bytes[right] {
            return false;
        }
        left += 1;
        right -= 1;
    }
    true
}

medium

A. Change right initialization to bytes.len() - 1

B. Change while condition to left <= right

C. Make left and right immutable

D. Use s.chars() instead of bytes

Solution

1. Step 1: Identify out-of-bounds error

   Right pointer is set to bytes.len(), which is out of valid index range (0 to len-1).

2. Step 2: Fix pointer initialization

   Set right to bytes.len() - 1 to point to last valid character index.

3. Final Answer:

   Change right initialization to bytes.len() - 1 -> Option A

4. Quick Check:

   Right pointer must be last index = Change right initialization to bytes.len() - 1 [OK]

Hint: Right pointer = length minus one to avoid out-of-bounds [OK]

Common Mistakes:

• Using length as index causes panic
• Changing while condition without fixing index
• Making pointers immutable prevents movement

5. Given the string s = "A man, a plan, a canal: Panama", which Rust code modification correctly checks if s is a palindrome ignoring non-alphanumeric characters and case using two pointers?

hard

A. Convert string to uppercase and compare characters including punctuation

B. Reverse the string and compare directly without filtering

C. Count characters ignoring case but include spaces and punctuation

D. Use two pointers skipping non-alphanumeric chars and compare lowercase characters

Solution

1. Step 1: Understand problem requirements

   We must ignore non-alphanumeric characters and case differences when checking palindrome.

2. Step 2: Apply two-pointer with skipping and case normalization

   Move pointers inward, skip chars that are not letters or digits, compare lowercase versions of characters.

3. Final Answer:

   Use two pointers skipping non-alphanumeric chars and compare lowercase characters -> Option D

4. Quick Check:

   Skip non-alphanumeric + lowercase compare = Use two pointers skipping non-alphanumeric chars and compare lowercase characters [OK]

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

Common Mistakes:

• Comparing with punctuation included
• Ignoring case differences
• Reversing string without filtering