Longest Palindromic Substring in DSA Rust - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to initialize the maximum palindrome length to 1.

DSA Rust

let mut max_len = [1];

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

A0

Busize::MAX

Cs.len()

D1

Attempts:

3 left

💡 Hint

Common Mistakes

Setting max_len to 0 which ignores single character palindromes.

Setting max_len to the string length initially.

The minimum palindrome length is 1 because a single character is always a palindrome.

2fill in blank

medium

Complete the code to expand around the center indices left and right.

DSA Rust

while left >= 0 && right < s.len() && s.chars().nth(left) == s.chars().nth([1]) {
    left -= 1;
    right += 1;
}

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

Aright

Bleft

Cleft + 1

Dright - 1

Attempts:

3 left

💡 Hint

Common Mistakes

Comparing the character at left with itself instead of right.

Using incorrect indices causing out of bounds.

We compare characters at left and right indices to check palindrome expansion.

3fill in blank

hard

Fix the error in updating the start index after finding a longer palindrome.

DSA Rust

if current_len > max_len {
    start = [1] + 1;
    max_len = current_len;
}

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

Aleft + 1

Bright

Cleft

Dright - 1

Attempts:

3 left

💡 Hint

Common Mistakes

Setting start to left directly causing off-by-one error.

Using right index which points after the palindrome.

After expanding, left is one step before the palindrome start, so we add 1.

4fill in blank

hard

Fill both blanks to correctly calculate the current palindrome length and update max_len.

DSA Rust

let current_len = right [1] left [2] 1;
if current_len > max_len {
    start = left + 1;
    max_len = current_len;
}

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

A-

B+

C*

D/

Attempts:

3 left

💡 Hint

Common Mistakes

Using +1 instead of -1.

Using multiplication or division instead of subtraction.

The length is calculated by subtracting left from right and subtracting 1.

5fill in blank

hard

Fill all three blanks to return the longest palindromic substring correctly.

DSA Rust

s[[1]..[2]].to_string() // returns substring from start to start + max_len

// Use slicing with correct indices

let result = &s[[1]..[2]];

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

Astart

Bstart + max_len

Cmax_len

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Using max_len alone as start index.

Using 0 as start index ignoring the found palindrome start.

The substring starts at 'start' and ends at 'start + max_len'.

Practice

1. What is the main goal of the Longest Palindromic Substring problem?

easy

A. Find the longest substring that contains only vowels

B. Find the longest substring with all unique characters

C. Find the longest substring with increasing characters

D. Find the longest substring that reads the same forwards and backwards

Solution

1. Step 1: Understand palindrome definition

   A palindrome reads the same forwards and backwards, like 'madam' or 'racecar'.

2. Step 2: Identify problem goal

   The problem asks to find the longest continuous part of a string that is a palindrome.

3. Final Answer:

   Find the longest substring that reads the same forwards and backwards -> Option D

4. Quick Check:

   Longest Palindromic Substring = Find longest palindrome [OK]

Hint: Look for substring that mirrors itself [OK]

Common Mistakes:

• Confusing substring with subsequence
• Looking for unique characters instead of palindrome
• Ignoring continuous substring requirement

2. Which Rust code snippet correctly declares a function header for finding the longest palindromic substring in a string slice s?

easy

A. fn longest_palindrome(s: &str) -> &str { }

B. fn longest_palindrome(s: String) -> String { }

C. fn longest_palindrome(s: &String) -> &String { }

D. fn longest_palindrome(s: str) -> str { }

Solution

1. Step 1: Understand Rust string types

   Rust uses &str for string slices and String for owned strings.

2. Step 2: Choose correct function signature

   Function should take a string slice &str and return a string slice &str for substring references.

3. Final Answer:

   fn longest_palindrome(s: &str) -> &str { } -> Option A

4. Quick Check:

   Use &str for slices in function signature [OK]

Hint: Use &str for input and output substrings [OK]

Common Mistakes:

• Using owned String when slice is better
• Returning owned String unnecessarily
• Incorrect parameter types like str without &

3. What will be the output of the following Rust code snippet that finds the longest palindromic substring?

fn main() {
    let s = "babad";
    let result = longest_palindrome(s);
    println!("{}", result);
}

fn longest_palindrome(s: &str) -> &str {
    let chars: Vec<char> = s.chars().collect();
    let mut start = 0;
    let mut max_len = 1;

    for i in 0..chars.len() {
        let mut left = i as i32;
        let mut right = i as i32;

        while left >= 0 && right < chars.len() as i32 && chars[left as usize] == chars[right as usize] {
            if (right - left + 1) > max_len {
                start = left as usize;
                max_len = (right - left + 1) as usize;
            }
            left -= 1;
            right += 1;
        }
    }

    &s[start..start + max_len]
}

medium

A. aba

B. bad

C. bab

D. babad

Solution

1. Step 1: Understand the code logic

   The code expands around each character to find odd-length palindromes and tracks the longest.

2. Step 2: Trace the input "babad"

   At index 0: palindrome "b" (length 1), index 1: "bab" (length 3), index 2: "aba" (length 3). The first longest palindrome found is "bab".

3. Final Answer:

   bab -> Option C

4. Quick Check:

   Longest palindrome substring = bab [OK]

Hint: Expand around center, pick first longest palindrome [OK]

Common Mistakes:

• Assuming palindrome must be unique
• Returning entire string instead of substring
• Confusing substring indices

4. Identify the bug in this Rust function that tries to find the longest palindromic substring:

fn longest_palindrome(s: &str) -> &str {
    let chars: Vec<char> = s.chars().collect();
    let mut start = 0;
    let mut max_len = 1;

    for i in 0..chars.len() {
        let mut left = i as i32;
        let mut right = i as i32 + 1;

        while left >= 0 && right < chars.len() as i32 && chars[left as usize] == chars[right as usize] {
            if (right - left + 1) > max_len {
                start = left as usize;
                max_len = (right - left + 1) as usize;
            }
            left -= 1;
            right += 1;
        }
    }

    &s[start..start + max_len]
}

medium

A. The max_len should start at 0, not 1

B. The initial right pointer should be equal to i, not i + 1

C. The loop should iterate from 1 to length, not 0 to length

D. The function should return a String, not &str

Solution

1. Step 1: Analyze pointer initialization

   Right pointer starts at i + 1, which only checks even-length palindromes, missing odd-length ones.

2. Step 2: Correct pointer for odd-length palindromes

   To check odd-length palindromes, right pointer should start at i, same as left pointer.

3. Final Answer:

   The initial right pointer should be equal to i, not i + 1 -> Option B

4. Quick Check:

   Right pointer = i for odd-length palindrome check [OK]

Hint: Start right pointer same as left for odd palindromes [OK]

Common Mistakes:

• Only checking even-length palindromes
• Wrong loop range causing index errors
• Returning wrong type causing borrow issues

5. Given the string "forgeeksskeegfor", which approach correctly finds the longest palindromic substring and what is that substring?

hard

A. Expand around center method; longest palindrome is "geeksskeeg"

B. Dynamic programming with substring table; longest palindrome is "forgeeksskeegfor"

C. Brute force checking all substrings; longest palindrome is "ss"

D. Using hash map for character counts; longest palindrome is "ks"

Solution

1. Step 1: Understand approaches to find longest palindrome

   Expand around center and dynamic programming are common methods; brute force is slow but correct.

2. Step 2: Identify longest palindrome in given string

   "forgeeksskeegfor" contains "geeksskeeg" as the longest palindrome substring, not the entire string.

3. Step 3: Match approach and answer

   Expand around center method efficiently finds "geeksskeeg"; dynamic programming would too but not the full string.

4. Final Answer:

   Expand around center method; longest palindrome is "geeksskeeg" -> Option A

5. Quick Check:

   Longest palindrome = geeksskeeg by center expansion [OK]

Hint: Expand around centers to find longest palindrome fast [OK]

Common Mistakes:

• Assuming entire string is palindrome
• Using character counts instead of substring checks
• Confusing brute force with efficient methods