Palindrome Detection in DSA Rust - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a palindrome?

A palindrome is a word, phrase, number, or sequence that reads the same backward as forward, ignoring spaces, punctuation, and capitalization.

beginner

How can you check if a string is a palindrome using two pointers?

Use two pointers: one at the start and one at the end of the string. Move both pointers towards the center, comparing characters. If all pairs match, the string is a palindrome.

intermediate

Why do we ignore case and non-alphanumeric characters when detecting palindromes?

Ignoring case and non-alphanumeric characters helps detect palindromes in phrases or sentences where spaces, punctuation, or letter case do not affect the palindrome property.

intermediate

What is the time complexity of palindrome detection using the two-pointer technique?

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

beginner

Show a simple Rust function signature for palindrome detection.

fn is_palindrome(s: &str) -> bool

Which of the following strings is a palindrome?

A world

B hello

C racecar

D rust

racecar reads the same backward and forward.

What technique uses two pointers to check palindrome?

A Divide and conquer

B Greedy algorithm

C Dynamic programming

D Two-pointer technique

Two-pointer technique compares characters from both ends moving inward.

What should be ignored when checking palindrome in sentences?

A Spaces and punctuation

B Only spaces

C Only punctuation

D Nothing

Spaces and punctuation do not affect palindrome property in sentences.

What is the time complexity of palindrome detection using two pointers?

A O(log n)

B O(n)

C O(n^2)

D O(1)

Each character is checked once, so time complexity is linear.

In Rust, what type is commonly used for input string in palindrome detection?

A &str

B String

C char

D Vec<char>

&str is a string slice, efficient for read-only string access.

Explain how the two-pointer technique works for palindrome detection.

Think about checking characters from both ends moving towards the center.

Describe why ignoring case and non-alphanumeric characters is important in palindrome detection for sentences.

Consider how phrases like 'A man, a plan, a canal: Panama' are palindromes.

Practice

1. What does a palindrome string mean in simple terms?

easy

A. It has all unique characters

B. It contains only vowels

C. It is always an even length string

D. It reads the same forwards and backwards

Solution

1. Step 1: Understand palindrome definition

   A palindrome is a word or phrase that reads the same from left to right and right to left.

2. Step 2: Check options against definition

   Only It reads the same forwards and backwards matches this meaning exactly.

3. Final Answer:

   It reads the same forwards and backwards -> Option D

4. Quick Check:

   Palindrome = Same forwards and backwards [OK]

Hint: Palindrome means mirror reading same both ways [OK]

Common Mistakes:

• Confusing palindrome with unique characters
• Thinking palindrome must be even length
• Assuming palindrome only has vowels

2. Which Rust code snippet correctly checks if a string s is a palindrome?

easy

A. s.reverse() == s

B. s.chars().eq(s.chars().rev())

C. s == s.reverse()

D. s.chars().all(|c| c == s.chars().next().unwrap())

Solution

1. Step 1: Understand Rust string palindrome check

   To check palindrome, compare characters iterator with its reverse iterator.

2. Step 2: Analyze options

   s.chars().eq(s.chars().rev()) uses chars().eq(chars().rev()) which correctly compares forward and backward characters. Options A and C use invalid methods or wrong comparisons. s.chars().all(|c| c == s.chars().next().unwrap()) checks if all characters are same, not palindrome.

3. Final Answer:

   s.chars().eq(s.chars().rev()) -> Option B

4. Quick Check:

   Compare chars iterator with reversed iterator [OK]

Hint: Use chars().eq(chars().rev()) to check palindrome [OK]

Common Mistakes:

• Using reverse() method on &str which does not exist
• Comparing string with reversed string incorrectly
• Checking all chars equal instead of palindrome

3. What is the output of this Rust code?

let s = "racecar";
let result = s.chars().eq(s.chars().rev());
println!("{}", result);

medium

A. true

B. Runtime panic

C. Compilation error

D. false

Solution

1. Step 1: Analyze the string and palindrome check

   The string "racecar" reads the same forwards and backwards, so it is a palindrome.

2. Step 2: Understand the code output

   The code compares characters forward and backward and returns true for palindrome strings. So, it prints true.

3. Final Answer:

   true -> Option A

4. Quick Check:

   "racecar" palindrome check = true [OK]

Hint: Check if string equals its reverse chars [OK]

Common Mistakes:

• Assuming palindrome check returns false for odd length
• Thinking code causes compile or runtime error
• Confusing output with string itself

4. Identify the error in this Rust palindrome check code:

fn is_palindrome(s: &str) -> bool {
    s == s.chars().rev().collect::<String>()
}

medium

A. Cannot compare &str with String directly

B. Missing semicolon after collect

C. chars().rev() does not exist

D. Function return type should be String

Solution

1. Step 1: Understand types in comparison

   The left side is &str, right side is String from collect(). Direct comparison between &str and String is not allowed.

2. Step 2: Fix type mismatch

   Convert left side to String or right side to &str for comparison. For example, use s.to_string() == s.chars().rev().collect::<String>().

3. Final Answer:

   Cannot compare &str with String directly -> Option A

4. Quick Check:

   Type mismatch in comparison causes error [OK]

Hint: Compare same types: &str vs &str or String vs String [OK]

Common Mistakes:

• Ignoring type mismatch error
• Assuming collect needs semicolon inside expression
• Thinking chars().rev() is invalid method

5. You want to detect palindromes ignoring case and spaces in Rust. Which code snippet correctly implements this?

hard

A. let is_pal = s.chars().all(|c| c.is_alphanumeric());

B. let lower = s.to_lowercase();
let is_pal = lower == lower.reverse();

C. let filtered: String = s.chars().filter(|c| !c.is_whitespace()).collect();
let lower = filtered.to_lowercase();
let is_pal = lower.chars().eq(lower.chars().rev());

D. let filtered: String = s.chars().filter(|c| c.is_whitespace()).collect();
let is_pal = filtered == filtered.chars().rev().collect::<String>();

Solution

1. Step 1: Remove spaces and convert to lowercase

   let filtered: String = s.chars().filter(|c| !c.is_whitespace()).collect(); let lower = filtered.to_lowercase(); let is_pal = lower.chars().eq(lower.chars().rev()); filters out whitespace, collects characters, then converts to lowercase string.

2. Step 2: Check palindrome on cleaned string

   It compares characters forward and backward correctly on the cleaned lowercase string.

3. Final Answer:

   let filtered: String = s.chars().filter(|c| !c.is_whitespace()).collect(); let lower = filtered.to_lowercase(); let is_pal = lower.chars().eq(lower.chars().rev()); -> Option C

4. Quick Check:

   Filter spaces + lowercase + compare reversed chars [OK]

Hint: Clean string first: remove spaces, lowercase, then check palindrome [OK]

Common Mistakes:

• Using reverse() method which does not exist
• Filtering only whitespace but not converting case
• Checking all chars alphanumeric instead of palindrome