Two Sum Problem Classic Hash Solution in DSA Rust - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a new empty hash map.

DSA Rust

let mut map = std::collections::HashMap::[1]();

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

Aget

Binsert

Cnew

Dremove

Attempts:

3 left

💡 Hint

Common Mistakes

Using methods like insert or get instead of new to create the map.

The new function creates an empty HashMap to store values.

2fill in blank

medium

Complete the code to check if the complement exists in the map.

DSA Rust

if let Some(&index) = map.[1](&complement) {

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

Aclear

Binsert

Cremove

Dget

Attempts:

3 left

💡 Hint

Common Mistakes

Using insert or remove instead of get to check for a key.

The get method checks if the complement key exists in the map.

3fill in blank

hard

Fix the error in the code to insert the current number and its index into the map.

DSA Rust

map.[1](nums[i], i);

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

Aget

Binsert

Cremove

Dclear

Attempts:

3 left

💡 Hint

Common Mistakes

Using get or remove instead of insert to add entries.

The insert method adds a key-value pair to the map.

4fill in blank

hard

Fill both blanks to complete the loop and calculate the complement.

DSA Rust

for [1] in 0..nums.len() {
    let complement = [2] - nums[i];

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

Ai

Btarget

Cj

Dsum

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong variable names for loop or complement calculation.

The loop variable is i and the complement is target - nums[i].

5fill in blank

hard

Fill all three blanks to return the indices of the two numbers that add up to the target.

DSA Rust

return vec![index, [1]];
}
map.[2](nums[[3]], [3]);

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

Ai

Binsert

Dget

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up insert and get methods or wrong variable names.

Return the current index i, insert the current number and index into the map.

Practice

1. What is the main purpose of using a hash map in the Two Sum problem classic hash solution?

easy

A. To reverse the array elements

B. To sort the array before searching

C. To store the sum of all pairs

D. To store numbers and their indices for quick lookup

Solution

1. Step 1: Understand the role of hash map

   The hash map stores each number as a key and its index as the value for quick access.

2. Step 2: Use hash map for complement lookup

   For each number, we check if the complement (target - number) exists in the hash map to find the pair.

3. Final Answer:

   To store numbers and their indices for quick lookup -> Option D

4. Quick Check:

   Hash map stores number:index pairs [OK]

Hint: Hash map helps find complements fast by storing indices [OK]

Common Mistakes:

• Thinking hash map sorts the array
• Trying to store sums instead of indices
• Using hash map to reverse elements

2. Which of the following Rust code snippets correctly declares a mutable hash map to solve the Two Sum problem?

easy

A. let map = HashMap::new();

B. let mut map = std::collections::HashMap::new();

C. let mut map = HashMap::new();

D. let map = std::collections::HashMap::new();

Solution

1. Step 1: Identify mutable hash map declaration

   To insert elements, the hash map must be mutable, so use 'let mut'.

2. Step 2: Use full path or import HashMap

   Using 'std::collections::HashMap::new()' is correct if HashMap is not imported.

3. Final Answer:

   let mut map = std::collections::HashMap::new(); -> Option B

4. Quick Check:

   Mutable hash map declaration requires 'mut' [OK]

Hint: Use 'let mut' to allow inserting into hash map [OK]

Common Mistakes:

• Forgetting 'mut' keyword for mutable map
• Using HashMap without importing or full path
• Declaring map immutable when insertions needed

3. What will be the output of the following Rust code snippet for the Two Sum problem?

fn two_sum(nums: Vec<i32>, target: i32) -> Vec<i32> {
    let mut map = std::collections::HashMap::new();
    for (i, &num) in nums.iter().enumerate() {
        if let Some(&index) = map.get(&(target - num)) {
            return vec![index as i32, i as i32];
        }
        map.insert(num, i);
    }
    vec![]
}

fn main() {
    let result = two_sum(vec![2, 7, 11, 15], 9);
    println!("{:?}", result);
}

medium

A. [2, 3]

B. [1, 0]

C. [0, 1]

D. []

Solution

1. Step 1: Iterate and check complements

   At i=0, num=2, map empty, insert (2,0). At i=1, num=7, complement=9-7=2 found at index 0.

2. Step 2: Return indices of complement pair

   Return vector with indices [0,1] as the pair sums to target 9.

3. Final Answer:

   [0, 1] -> Option C

4. Quick Check:

   Indices of 2 and 7 are [0,1] [OK]

Hint: Check complement in map before inserting current number [OK]

Common Mistakes:

• Returning indices in wrong order
• Not casting usize to i32 for return
• Returning empty vector when pair exists

4. Identify the bug in this Rust Two Sum solution and select the fix:

fn two_sum(nums: Vec<i32>, target: i32) -> Vec<i32> {
    let mut map = std::collections::HashMap::new();
    for (i, num) in nums.iter().enumerate() {
        if let Some(&index) = map.get(&(target - num)) {
            return vec![index as i32, i as i32];
        }
        map.insert(num, i);
    }
    vec![]
}

medium

A. Change 'num' to '*num' in map.insert to match key type

B. Remove 'as i32' casts from return vector

C. Change 'map.get(&(target - num))' to 'map.get(&(target + num))'

D. Declare map as immutable instead of mutable

Solution

1. Step 1: Check key types in hash map

   Keys are i32 values, but 'num' is a reference (&i32), so insert needs dereferenced value.

2. Step 2: Fix insertion to use dereferenced num

   Change 'map.insert(num, i);' to 'map.insert(*num, i);' to insert i32 key, matching get() key type.

3. Final Answer:

   Change 'num' to '*num' in map.insert to match key type -> Option A

4. Quick Check:

   Dereference num when inserting key [OK]

Hint: Dereference iterator item before inserting into hash map [OK]

Common Mistakes:

• Inserting reference instead of value as key
• Changing target arithmetic incorrectly
• Making map immutable causing insert error

5. Given the input array [3, 3, 4, 5] and target 6, what will the Two Sum classic hash solution return and why?

hard

A. [0, 1] because the first two 3's sum to 6

B. [1, 2] because 3 and 4 sum to 7, closest to 6

C. [2, 3] because 4 and 5 sum to 9, which is greater than 6

D. [] because no two numbers sum exactly to 6

Solution

1. Step 1: Check pairs for target sum

   The first element 3 at index 0 is inserted. At index 1, num=3, complement=6-3=3 found at index 0.

2. Step 2: Return indices of first matching pair

   Return [0,1] because these two 3's sum exactly to 6.

3. Final Answer:

   [0, 1] because the first two 3's sum to 6 -> Option A

4. Quick Check:

   First pair summing to target returned [OK]

Hint: Return first found pair that sums to target [OK]

Common Mistakes:

• Choosing pairs that don't sum to target
• Ignoring duplicate values as valid pairs
• Returning empty vector incorrectly