Two Sum Problem Classic Hash Solution in DSA Rust - Build from Scratch

Two Sum Problem Classic Hash Solution

📖 Scenario: You are building a simple tool to find two numbers in a list that add up to a target number. This is useful in many real-life cases like splitting bills or finding matching pairs in shopping.

🎯 Goal: Build a Rust program that finds the first pair of indices of two numbers in a list that add up to a given target using a hash map for fast lookup.

📋 What You'll Learn

Create a vector called nums with the exact values [2, 7, 11, 15]

Create an integer variable called target and set it to 9

Use a HashMap called seen to store numbers and their indices

Use a for loop with variables i and num to iterate over nums.iter().enumerate()

Inside the loop, check if target - num exists in seen and return the pair of indices

Print the result vector of indices

💡 Why This Matters

🌍 Real World

Finding pairs of numbers that add up to a target is useful in finance, shopping discounts, and data analysis.

💼 Career

This problem is a common coding interview question and helps understand hash maps and efficient searching.

1

Create the number list

Create a vector called nums with the exact values [2, 7, 11, 15]

DSA Rust

# Create the vector nums with values [2, 7, 11, 15]
# Your code here

Hint

Use let nums = vec![2, 7, 11, 15]; to create the vector.

2

Set the target value

Create an integer variable called target and set it to 9

DSA Rust

fn main() {
    let nums = vec![2, 7, 11, 15];
    // Create target variable with value 9
    # Your code here
}

Hint

Use let target = 9; to set the target number.

3

Find two sum indices using a hash map

Use a HashMap called seen to store numbers and their indices. Use a for loop with variables i and num to iterate over nums.iter().enumerate(). Inside the loop, check if target - num exists in seen and return the pair of indices in a vector called result. If not found, insert num and i into seen. Initialize result as an empty vector before the loop.

DSA Rust

use std::collections::HashMap;

fn main() {
    let nums = vec![2, 7, 11, 15];
    let target = 9;
    // Create HashMap seen and vector result
    // Use for loop with i, num in nums.iter().enumerate()
    // Check if target - num is in seen
    // If yes, set result to vec![seen_index, i] and break
    // Else insert num and i into seen
    # Your code here
}

Hint

Use HashMap::new() to create seen. Use nums.iter().enumerate() in the loop. Use seen.get(&complement) to check for complement.

4

Print the result

Print the vector result using println!("{:?}", result)

DSA Rust

use std::collections::HashMap;

fn main() {
    let nums = vec![2, 7, 11, 15];
    let target = 9;
    let mut seen = HashMap::new();
    let mut result = Vec::new();
    for (i, num) in nums.iter().enumerate() {
        let complement = target - *num;
        if let Some(&index) = seen.get(&complement) {
            result = vec![index, i];
            break;
        }
        seen.insert(*num, i);
    }
    // Print the result vector
    # Your code here
}

Hint

Use println!("{:?}", result); to print the vector.

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