Valid Palindrome Two Pointer in DSA Rust - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

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

DSA Rust

let mut left = [1];

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

As.len()

B1

C0

Ds.len() - 1

Attempts:

3 left

💡 Hint

Common Mistakes

Starting left pointer at 1 skips the first character.

Starting at s.len() is out of bounds.

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 Rust

let mut right = [1];

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

A0

B1

Cs.len()

Ds.len() - 1

Attempts:

3 left

💡 Hint

Common Mistakes

Using s.len() causes out of bounds error.

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

The right pointer should start at the last valid index, which is length minus one.

3fill in blank

hard

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

DSA Rust

while [1] < right {

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

Aleft

Bleft + 1

Cright

Dright - 1

Attempts:

3 left

💡 Hint

Common Mistakes

Using right < right is always false.

Using left + 1 < right skips first comparison.

The loop should run while left pointer is less than right pointer to compare characters.

4fill in blank

hard

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

DSA Rust

while left < right && !s.chars().nth(left).unwrap().is_alphanumeric() [1];
while left < right && !s.chars().nth(right).unwrap().is_alphanumeric() [2];

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

Aleft += 1

Bright -= 1

Cleft -= 1

Dright += 1

Attempts:

3 left

💡 Hint

Common Mistakes

Decrementing left pointer causes underflow.

Incrementing right pointer goes out of bounds.

Increment left pointer to skip non-alphanumeric from start, decrement right pointer to skip from end.

5fill in blank

hard

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

DSA Rust

if s.chars().nth(left).unwrap().to_ascii_lowercase() != s.chars().nth(right).unwrap().to_ascii_lowercase() {
    return false;
}
left [1];
right [2];
}
return [3];

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

A+= 1

B-= 1

Ctrue

Dfalse

Attempts:

3 left

💡 Hint

Common Mistakes

Returning false at the end instead of true.

Moving pointers in wrong directions.

Move left pointer forward, right pointer backward, and return true if all matched.

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