String Reversal Approaches in DSA Rust - Practice Problems & Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of reversing a string using chars().rev().collect()

What is the output of this Rust code snippet that reverses a string?

DSA Rust

fn main() {
    let original = "hello";
    let reversed: String = original.chars().rev().collect();
    println!("{}", reversed);
}

A olleh

B hello

C olleh\n

D SyntaxError

💡 Hint

chars() returns characters, rev() reverses iterator, collect() gathers into String.

The code reverses the characters of the string "hello" and prints it. The output is "olleh" without extra newline characters in the string itself.

❓ Predict Output

intermediate

Output of reversing a string using bytes().rev().collect()

What happens when reversing a string using bytes().rev().collect() in Rust?

DSA Rust

fn main() {
    let original = "hello";
    let reversed: Vec<u8> = original.bytes().rev().collect();
    let reversed_str = String::from_utf8(reversed).unwrap();
    println!("{}", reversed_str);
}

A Runtime error due to invalid UTF-8

B hello

C olleh

D SyntaxError

💡 Hint

bytes() returns bytes, rev() reverses bytes, from_utf8 converts bytes back to string.

Since "hello" is ASCII, reversing bytes is same as reversing characters. The output is "olleh".

❓ Predict Output

advanced

Output of reversing a Unicode string with chars().rev().collect()

What is the output of reversing the Unicode string "héllo" using chars().rev().collect() in Rust?

DSA Rust

fn main() {
    let original = "héllo";
    let reversed: String = original.chars().rev().collect();
    println!("{}", reversed);
}

A héllo

B olléh\n

C SyntaxError

D olléh

💡 Hint

chars() handles Unicode characters correctly.

The code reverses the Unicode string correctly by characters, so the accented 'é' stays intact and reversed properly.

❓ Predict Output

advanced

What error occurs when reversing a string using bytes().rev().collect() on Unicode?

What happens when reversing the Unicode string "héllo" using bytes().rev().collect() and converting to String in Rust?

DSA Rust

fn main() {
    let original = "héllo";
    let reversed: Vec<u8> = original.bytes().rev().collect();
    let reversed_str = String::from_utf8(reversed).unwrap();
    println!("{}", reversed_str);
}

A Runtime panic due to invalid UTF-8 sequence

B olléh

C héllo

D SyntaxError

💡 Hint

Reversing bytes of multi-byte Unicode characters breaks UTF-8 encoding.

Reversing bytes of Unicode string breaks multi-byte characters, causing invalid UTF-8 and panic on from_utf8().

🧠 Conceptual

expert

Why is chars().rev().collect() preferred over bytes().rev().collect() for string reversal in Rust?

Choose the best reason why reversing a string with chars().rev().collect() is safer than using bytes().rev().collect() in Rust.

A bytes() is slower than chars() because it processes each byte individually, while chars() processes the whole string at once.

B chars() reverses Unicode characters correctly, preserving valid UTF-8, while bytes() reverses raw bytes which can break multi-byte characters.

C chars() uses less memory than bytes() because it skips ASCII characters, while bytes() stores all bytes including spaces.

D bytes() automatically fixes invalid UTF-8 sequences after reversal, while chars() does not.

💡 Hint

Think about how Unicode characters are stored and how reversing bytes affects them.

chars() iterates over Unicode scalar values, so reversing preserves character integrity. bytes() reverses raw bytes, which can break multi-byte characters and cause invalid UTF-8.

Practice

1. What does the chars().rev().collect() method chain do when applied to a Rust string?

easy

A. It reverses the characters of the string and collects them into a new string.

B. It sorts the characters of the string in ascending order.

C. It removes all whitespace characters from the string.

D. It converts the string to uppercase.

Solution

1. Step 1: Understand chars() method

   The chars() method converts the string into an iterator over its characters.

2. Step 2: Apply rev() and collect()

   The rev() method reverses the iterator, and collect() gathers the reversed characters into a new string.

3. Final Answer:

   It reverses the characters of the string and collects them into a new string. -> Option A

4. Quick Check:

   chars().rev().collect() reverses string [OK]

Hint: Remember: chars() + rev() + collect() reverses a string [OK]

Common Mistakes:

• Thinking it sorts characters instead of reversing
• Confusing rev() with removing characters
• Assuming it changes case of letters

2. Which of the following Rust code snippets correctly reverses a string s?

easy

A. let reversed: String = s.reverse();

B. let reversed: String = s.chars().rev().collect();

C. let reversed: String = s.collect().rev().chars();

D. let reversed: String = s.chars().collect().rev();

Solution

1. Step 1: Check method availability

   Rust strings do not have a reverse() method, so let reversed: String = s.reverse(); is invalid.

2. Step 2: Analyze method chaining

   let reversed: String = s.chars().rev().collect(); uses chars() to get characters, rev() to reverse, and collect() to form a new string, which is correct.

3. Final Answer:

   let reversed: String = s.chars().rev().collect(); -> Option B

4. Quick Check:

   Correct syntax for string reversal is let reversed: String = s.chars().rev().collect(); [OK]

Hint: Use chars().rev().collect() to reverse strings in Rust [OK]

Common Mistakes:

• Using non-existent reverse() method on String
• Wrong order of method calls
• Trying to reverse after collecting into String

3. What is the output of the following Rust code?

let s = String::from("rust");
let reversed: String = s.chars().rev().collect();
println!("{}", reversed);

medium

A. Error: cannot reverse string

B. rust

C. tsur

D. tusr

Solution

1. Step 1: Understand input string

   The string "rust" has characters 'r', 'u', 's', 't'.

2. Step 2: Reverse characters and collect

   Using chars().rev().collect() reverses the characters to 't', 's', 'u', 'r' and collects into a new string.

3. Final Answer:

   tsur -> Option C

4. Quick Check:

   Reversed "rust" = "tsur" [OK]

Hint: Trace chars().rev().collect() to get reversed string output [OK]

Common Mistakes:

• Expecting original string output
• Mixing character order incorrectly
• Assuming runtime error on reversal

4. Identify the error in this Rust code that attempts to reverse a string:

let s = String::from("hello");
let reversed = s.reverse();
println!("{}", reversed);

medium

A. reverse() returns a vector, not a string

B. Missing semicolon after println! macro

C. Cannot print reversed string directly

D. String type has no method named reverse()

Solution

1. Step 1: Check method existence on String

   Rust's String type does not have a reverse() method, so calling it causes a compile error.

2. Step 2: Confirm other syntax correctness

   The semicolon after println! is present, and printing a string is valid, so no errors there.

3. Final Answer:

   String type has no method named reverse() -> Option D

4. Quick Check:

   reverse() method does not exist on String [OK]

Hint: Remember: String has no reverse() method in Rust [OK]

Common Mistakes:

• Assuming reverse() exists on String
• Thinking reverse() returns vector automatically
• Ignoring compiler error messages

5. You want to reverse a string but keep all Unicode grapheme clusters intact (like emojis or accented letters). Which Rust crate and approach is best suited for this?

hard

A. Use the unicode-segmentation crate to split by graphemes, then reverse and collect.

B. Use chars().rev().collect() directly on the string.

C. Convert string to bytes, reverse bytes, then convert back to string.

D. Use split_whitespace() and reverse the words.

Solution

1. Step 1: Understand Unicode grapheme clusters

   Some characters like emojis or accented letters are multiple Unicode code points but one visible character (grapheme cluster).

2. Step 2: Use appropriate crate and method

   The unicode-segmentation crate provides graphemes() method to split string by grapheme clusters, which can then be reversed safely.

3. Final Answer:

   Use the unicode-segmentation crate to split by graphemes, then reverse and collect. -> Option A

4. Quick Check:

   unicode-segmentation crate handles grapheme reversal correctly [OK]

Hint: Use unicode-segmentation crate for correct Unicode string reversal [OK]

Common Mistakes:

• Reversing by chars() breaks multi-codepoint characters
• Reversing bytes corrupts UTF-8 encoding
• Reversing words instead of characters