Count Words with Given Prefix in DSA Rust - Practice Problems & Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of prefix count after inserting words

What is the output of the Rust code that inserts words into a prefix count structure and queries the count for prefix "app"?

DSA Rust

struct TrieNode {
    children: std::collections::HashMap<char, TrieNode>,
    count: usize,
}

impl TrieNode {
    fn new() -> Self {
        TrieNode {
            children: std::collections::HashMap::new(),
            count: 0,
        }
    }

    fn insert(&mut self, word: &str) {
        let mut node = self;
        for ch in word.chars() {
            node = node.children.entry(ch).or_insert_with(TrieNode::new);
            node.count += 1;
        }
    }

    fn count_prefix(&self, prefix: &str) -> usize {
        let mut node = self;
        for ch in prefix.chars() {
            if let Some(next) = node.children.get(&ch) {
                node = next;
            } else {
                return 0;
            }
        }
        node.count
    }
}

fn main() {
    let mut root = TrieNode::new();
    root.insert("apple");
    root.insert("app");
    root.insert("application");
    root.insert("apt");
    println!("{}", root.count_prefix("app"));
}

A 4

B 2

C 3

D 1

💡 Hint

Count how many inserted words start with "app".

The words "apple", "app", and "application" all start with "app", so the count is 3.

🧠 Conceptual

intermediate

Understanding prefix count in a trie

If a trie stores the words ["cat", "car", "cart", "dog"], what is the count of words with prefix "car"?

A 3

B 2

C 1

D 0

💡 Hint

Count words starting exactly with "car" prefix.

The words "car" and "cart" start with "car", so the count is 2.

🔧 Debug

advanced

Identify the error in prefix count code

What error does this Rust code produce when counting prefix "ba" after inserting words "bat" and "ball"? Code snippet: let mut root = TrieNode::new(); root.insert("bat"); root.insert("ball"); println!("{}", root.count_prefix("ba")); struct TrieNode { children: std::collections::HashMap, count: usize, } impl TrieNode { fn new() -> Self { TrieNode { children: std::collections::HashMap::new(), count: 0, } } fn insert(&mut self, word: &str) { let mut node = self; for ch in word.chars() { node = node.children.entry(ch).or_insert_with(TrieNode::new); // Missing count increment here } } fn count_prefix(&self, prefix: &str) -> usize { let mut node = self; for ch in prefix.chars() { if let Some(next) = node.children.get(&ch) { node = next; } else { return 0; } } node.count } }

A Runtime panic

B 2

C Compilation error

D 0

💡 Hint

Check if counts are updated during insertion.

The count field is never incremented during insertion, so it remains zero for all nodes. Thus, count_prefix returns 0.

❓ Predict Output

advanced

Output after multiple prefix queries

What is the output of the following Rust code printing counts for prefixes "a", "ap", "app", and "b"?

DSA Rust

struct TrieNode {
    children: std::collections::HashMap<char, TrieNode>,
    count: usize,
}

impl TrieNode {
    fn new() -> Self {
        TrieNode {
            children: std::collections::HashMap::new(),
            count: 0,
        }
    }

    fn insert(&mut self, word: &str) {
        let mut node = self;
        for ch in word.chars() {
            node = node.children.entry(ch).or_insert_with(TrieNode::new);
            node.count += 1;
        }
    }

    fn count_prefix(&self, prefix: &str) -> usize {
        let mut node = self;
        for ch in prefix.chars() {
            if let Some(next) = node.children.get(&ch) {
                node = next;
            } else {
                return 0;
            }
        }
        node.count
    }
}

fn main() {
    let mut root = TrieNode::new();
    root.insert("apple");
    root.insert("app");
    root.insert("ape");
    root.insert("bat");
    println!("{} {} {} {}", root.count_prefix("a"), root.count_prefix("ap"), root.count_prefix("app"), root.count_prefix("b"));
}

A 4 3 2 1

B 3 3 2 1

C 4 2 2 1

D 3 2 1 1

💡 Hint

Count words starting with each prefix carefully.

Words starting with "a" are "apple", "app", "ape" (3 words). But count includes nodes, so counts are cumulative: root->a (4), a->p (3), ap->p (2), b (1).

🚀 Application

expert

Number of words with prefix after deletions

Given a trie that supports insertions and deletions, after inserting words ["test", "team", "teach", "tear"] and deleting "team", what is the count of words with prefix "te"?

DSA Rust

struct TrieNode {
    children: std::collections::HashMap<char, TrieNode>,
    count: usize,
    end_count: usize,
}

impl TrieNode {
    fn new() -> Self {
        TrieNode {
            children: std::collections::HashMap::new(),
            count: 0,
            end_count: 0,
        }
    }

    fn insert(&mut self, word: &str) {
        let mut node = self;
        for ch in word.chars() {
            node = node.children.entry(ch).or_insert_with(TrieNode::new);
            node.count += 1;
        }
        node.end_count += 1;
    }

    fn delete(&mut self, word: &str) -> bool {
        if !self._delete(word, 0) {
            return false;
        }
        true
    }

    fn _delete(&mut self, word: &str, index: usize) -> bool {
        if index == word.len() {
            if self.end_count == 0 {
                return false;
            }
            self.end_count -= 1;
            self.count -= 1;
            return true;
        }
        let ch = word.chars().nth(index).unwrap();
        if let Some(child) = self.children.get_mut(&ch) {
            if child._delete(word, index + 1) {
                self.count -= 1;
                if child.count == 0 {
                    self.children.remove(&ch);
                }
                return true;
            }
        }
        false
    }

    fn count_prefix(&self, prefix: &str) -> usize {
        let mut node = self;
        for ch in prefix.chars() {
            if let Some(next) = node.children.get(&ch) {
                node = next;
            } else {
                return 0;
            }
        }
        node.count
    }
}

fn main() {
    let mut root = TrieNode::new();
    root.insert("test");
    root.insert("team");
    root.insert("teach");
    root.insert("tear");
    root.delete("team");
    println!("{}", root.count_prefix("te"));
}

A 3

B 4

C 2

D 1

💡 Hint

Count words starting with "te" after removing "team".

After deleting "team", the words with prefix "te" are "test", "teach", and "tear", so count is 3.

Practice

1. What does the problem "Count Words with Given Prefix" ask you to do?

easy

A. Find the longest word in a list

B. Count how many words in a list start with a specific prefix

C. Count the total number of characters in all words

D. Sort the words alphabetically

Solution

1. Step 1: Understand the problem statement

   The problem asks to find how many words start with a given prefix, not about length or sorting.

2. Step 2: Identify the key task

   Counting words that begin with the prefix is the main task.

3. Final Answer:

   Count how many words in a list start with a specific prefix -> Option B

4. Quick Check:

   Count words starting with prefix = Count how many words in a list start with a specific prefix [OK]

Hint: Focus on words starting with prefix, not full word or sorting [OK]

Common Mistakes:

• Confusing prefix with substring anywhere in word
• Counting total characters instead of words
• Sorting words instead of counting

2. Which of the following Rust code snippets correctly checks if a word starts with a given prefix prefix?

easy

A. word.starts_with(prefix)

B. word.contains(prefix)

C. word.ends_with(prefix)

D. word == prefix

Solution

1. Step 1: Recall Rust string methods

   Rust's starts_with method checks if a string begins with a given substring.

2. Step 2: Compare options

   contains checks anywhere, ends_with checks suffix, and == checks full equality, so only starts_with fits.

3. Final Answer:

   word.starts_with(prefix) -> Option A

4. Quick Check:

   Use starts_with() to check prefix [OK]

Hint: Use starts_with() method to check prefix in Rust strings [OK]

Common Mistakes:

• Using contains() which matches anywhere, not just prefix
• Using ends_with() which checks suffix
• Comparing full equality instead of prefix

3. What is the output of this Rust code?

let words = vec!["apple", "app", "application", "banana"];
let prefix = "app";
let count = words.iter().filter(|w| w.starts_with(prefix)).count();
println!("{}", count);

medium

A. 3

B. 2

C. 1

D. 4

Solution

1. Step 1: Identify words starting with "app"

   Words are "apple", "app", "application", and "banana". The first three start with "app".

2. Step 2: Count matching words

   Count is 3 because "apple", "app", and "application" start with "app".

3. Final Answer:

   3 -> Option A

4. Quick Check:

   3 words start with "app" [OK]

Hint: Count words where starts_with(prefix) is true [OK]

Common Mistakes:

• Counting only exact matches
• Including words that don't start with prefix
• Off-by-one counting errors

4. The following Rust code tries to count words starting with a prefix but gives wrong output. What is the bug?

let words = vec!["cat", "car", "dog", "scare"];
let prefix = "car";
let count = words.iter().filter(|w| w.contains(prefix)).count();
println!("{}", count);

medium

A. Missing semicolon after count declaration

B. Prefix variable is not declared correctly

C. The vector words is empty

D. Using contains() instead of starts_with() causes wrong matches

Solution

1. Step 1: Understand difference between contains() and starts_with()

   contains() checks if prefix appears anywhere, not just at start.

2. Step 2: Identify incorrect matches

   "scare" contains "car" but does not start with "car", so contains() wrongly counts it.

3. Final Answer:

   Using contains() instead of starts_with() causes wrong matches -> Option D

4. Quick Check:

   Use starts_with() to count prefix matches [OK]

Hint: Replace contains() with starts_with() to fix prefix counting [OK]

Common Mistakes:

• Confusing contains() with starts_with()
• Assuming contains() only matches prefix
• Ignoring method differences

5. Given a list of words ["test", "tester", "testing", "toast", "team"] and prefix "te", which Rust code correctly counts words starting with the prefix and ignores case sensitivity?

hard

A. words.iter().filter(|w| w.contains(prefix)).count()

B. words.iter().filter(|w| w.starts_with(prefix)).count()

C. words.iter().filter(|w| w.to_lowercase().starts_with(&prefix.to_lowercase())).count()

D. words.iter().filter(|w| w.ends_with(prefix)).count()

Solution

1. Step 1: Understand case sensitivity in Rust strings

   Rust's starts_with is case sensitive, so we convert both word and prefix to lowercase.

2. Step 2: Check each option for case-insensitive prefix check

   words.iter().filter(|w| w.to_lowercase().starts_with(&prefix.to_lowercase())).count() converts both to lowercase before checking prefix, ensuring case-insensitive match. Others do not handle case.

3. Final Answer:

   words.iter().filter(|w| w.to_lowercase().starts_with(&prefix.to_lowercase())).count() -> Option C

4. Quick Check:

   Convert both strings to lowercase before starts_with() [OK]

Hint: Lowercase both word and prefix before starts_with() for case-insensitive [OK]

Common Mistakes:

• Ignoring case sensitivity in starts_with()
• Using contains() which is wrong for prefix
• Using ends_with() instead of starts_with()