Solve the Easy-level Two Sum coding interview problem online in Python & JavaScript. Run your code against real test cases in the browser and study worked examples. It covers arrays and hashmap. Read the step-by-step solution and complexity analysis. Java starter code is also included for reference (no in-browser JVM yet, so it can't be run here).

Two Sum – Solve Online in Python & JavaScript

Two Sum: finding two numbers in an array that add up to a given target number

Given an array of integers nums and an integer target, ==return indices of the two numbers== such that they add up to target.

You may assume that ==each input would have exactly one solution==, and you ==may not use the same element twice==.

You can return the answer in any order.

- You are given an array of integers and a target sum.
- The task is to find two distinct numbers in the array that add up to the target.
- You should return their indices as a result.

Understanding the Problem

- Start by considering a brute force approach where you check every possible pair.
- For each number, iterate through the remaining numbers and check if they add up to the target.
- This approach has O(n²) time complexity.

Brute Force Approach (Inefficient)

- The core operation is checking whether two numbers in the array add up to the target.
- For each number `num`, check if there’s another number `complement` such that:
  `num + complement = target`
- The complement can be calculated as `complement = target - num`.

Identifying the Core Operation

- Searching through the array takes O(n) time if done linearly. To make this faster, you can use a hash map (dictionary).
- A hash map allows for constant-time lookups (O(1)), so you can check if the complement exists as you iterate through the array.

Using a Hash Map for Efficient Lookup

- Create an empty hash map to store numbers and their indices as you iterate through the array.

Initializing the Hash Map

- Loop through the array.
- For each number, calculate its complement (the difference between the target and the current number).
- The complement is calculated as `target - num`.

Iterate Through the Array and Calculate Complement

- For each number, check if the complement exists in the hash map.
- If the complement is already in the hash map, it means you’ve found two numbers that add up to the target.
- Return the indices of the current number and its complement.

Check if the Complement Exists in the Hash Map

- If the complement is not found in the hash map, store the current number and its index in the hash map.
- This ensures that it can be checked as a complement for future numbers.

Store the Current Number in the Hash Map

- Ensure that the input array has at least two numbers.
- Return `None` if no solution is found.