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

Two Sum Problem Classic Hash Solution

📖 Scenario: You are helping a friend who runs a small store. They want to find two items from their price list that add up to a specific gift card amount. This will help customers quickly find pairs of items they can buy together.

🎯 Goal: Build a program that finds two numbers in a list that add up to a target sum using a hash map (object) for fast lookup.

📋 What You'll Learn

Create an array called prices with the exact values: [2, 7, 11, 15]

Create a variable called target and set it to 9

Use a for loop with variable i to iterate over prices

Inside the loop, use a hash map object called seen to check complements

Print the indices of the two numbers that add up to target as an array

💡 Why This Matters

🌍 Real World

Finding two items that sum to a gift card value helps customers shop quickly and improves store sales.

💼 Career

This problem is a classic example of using hash maps for fast lookup, a key skill in software development and coding interviews.

1

Create the prices array

Create an array called prices with these exact values: [2, 7, 11, 15]

DSA Typescript

// Create the prices array with the given values
// Your code here

Hint

Use const prices = [2, 7, 11, 15]; to create the array.

2

Set the target sum

Create a variable called target and set it to 9

DSA Typescript

const prices = [2, 7, 11, 15];
// Create the target variable with value 9
// Your code here

Hint

Use const target = 9; to set the target sum.

3

Find two indices using a hash map

Create an empty object called seen. Use a for loop with variable i to iterate over prices. Inside the loop, calculate complement = target - prices[i]. If complement is a key in seen, break the loop. Otherwise, add prices[i] as a key with value i to seen.

DSA Typescript

const prices = [2, 7, 11, 15];
const target = 9;
// Create seen object and for loop to find two indices
// Your code here

Hint

Use an object to store seen numbers and their indices. Check if complement exists before adding current number.

4

Print the result

Print the result array using console.log(result)

DSA Typescript

const prices = [2, 7, 11, 15];
const target = 9;
const seen: {[key: number]: number} = {};
let result: number[] = [];
for (let i = 0; i < prices.length; i++) {
    const complement = target - prices[i];
    if (complement in seen) {
        result = [seen[complement], i];
        break;
    }
    seen[prices[i]] = i;
}
// Print the result array
// Your code here

Hint

Use console.log(result); to show the indices.

Practice

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

easy

A. To store the sum of all pairs

B. To sort the array before searching

C. To store numbers and their indices for quick lookup

D. To reverse the array elements

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 lookup.

2. Step 2: Explain why quick lookup is needed

   Quick lookup helps to find if the complement (target - current number) exists without scanning the entire array again.

3. Final Answer:

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

4. Quick Check:

   Hash map stores number:index = A [OK]

Hint: Hash map stores values for instant complement check [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 TypeScript code snippets correctly initializes an empty hash map for the Two Sum problem?

easy

A. const map = new Map();

B. const map = {};

C. const map = new Array();

D. const map = new Set();

Solution

1. Step 1: Identify correct hash map initialization

   In TypeScript, new Map<number, number>() creates a map with number keys and values.

2. Step 2: Explain why other options are incorrect

   {} is an object but lacks type safety; new Array() is an array, not a map; new Set() stores unique values, not key-value pairs.

3. Final Answer:

   const map = new Map<number, number>(); -> Option A

4. Quick Check:

   Use Map<number, number> for hash map [OK]

Hint: Use Map<number, number> for key-value pairs [OK]

Common Mistakes:

• Using Set instead of Map
• Using Array instead of Map
• Using plain object without type safety

3. What will be the output of the following TypeScript code for the input array [2, 7, 11, 15] and target 9?

function twoSum(nums: number[], target: number): number[] {
  const map = new Map();
  for (let i = 0; i < nums.length; i++) {
    const complement = target - nums[i];
    if (map.has(complement)) {
      return [map.get(complement)!, i];
    }
    map.set(nums[i], i);
  }
  return [];
}

console.log(twoSum([2, 7, 11, 15], 9));

medium

A. [1, 2]

B. []

C. [2, 3]

D. [0, 1]

Solution

1. Step 1: Trace the loop and map updates

   i=0: map={}, complement=9-2=7, map.has(7)? No, map.set(2,0).
   i=1: complement=9-7=2, map.has(2)? Yes, index=0, return [0,1].

2. Step 2: Confirm returned indices

   Indices 0 and 1 correspond to numbers 2 and 7 which sum to 9.

3. Final Answer:

   [0, 1] -> Option D

4. Quick Check:

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

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

Common Mistakes:

• Returning values instead of indices
• Not checking map before insertion
• Returning empty array incorrectly

4. Identify the bug in this TypeScript Two Sum implementation:

function twoSum(nums: number[], target: number): number[] {
  const map = new Map();
  for (let i = 0; i <= nums.length; i++) {
    const complement = target - nums[i];
    if (map.has(complement)) {
      return [map.get(complement)!, i];
    }
    map.set(nums[i], i);
  }
  return [];
}

medium

A. map.set should be called before checking complement

B. Loop condition should be < instead of <=

C. Return statement should return values instead of indices

D. Complement calculation is incorrect

Solution

1. Step 1: Analyze loop condition

   The loop uses i <= nums.length, which causes out-of-bounds access at last iteration.

2. Step 2: Correct loop condition

   Change to i < nums.length to avoid accessing undefined element.

3. Final Answer:

   Loop condition should be < instead of <= -> Option B

4. Quick Check:

   Loop must stay within array bounds [OK]

Hint: Use < not <= for array index loops [OK]

Common Mistakes:

• Off-by-one error in loop condition
• Swapping order of map.set and map.has
• Returning values instead of indices

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

function twoSum(nums: number[], target: number): number[] {
  const map = new Map();
  for (let i = 0; i < nums.length; i++) {
    const complement = target - nums[i];
    if (map.has(complement)) {
      return [map.get(complement)!, i];
    }
    map.set(nums[i], i);
  }
  return [];
}

console.log(twoSum([3, 3, 4, 5], 6));

hard

A. [0, 1]

B. [1, 2]

C. [2, 3]

D. []

Solution

1. Step 1: Trace the loop and map updates with duplicates

   i=0: map={}, complement=6-3=3, map.has(3)? No, map.set(3,0).
   i=1: complement=6-3=3, map.has(3)? Yes, index=0, return [0,1].

2. Step 2: Confirm correct pair with duplicates

   Even with duplicates, first pair indices 0 and 1 sum to 6.

3. Final Answer:

   [0, 1] -> Option A

4. Quick Check:

   Duplicates handled, indices [0,1] sum to target [OK]

Hint: Duplicates allowed; first matching pair returned [OK]

Common Mistakes:

• Ignoring duplicates and returning wrong indices
• Returning empty array incorrectly
• Confusing values with indices