Is Two Sum easy or hard?

Two Sum is categorized as an Easy problem. The brute force logic is straightforward, but the key learning objective is recognizing how a hash table reduces the search from O(n^2) to O(n). It is often the first problem used to introduce hash map optimization patterns.

How to solve Two Sum in O(n)?

Use a hash map to store numbers you have already visited along with their indices. While iterating through the array, compute the complement as target minus the current value. If the complement exists in the map, return the stored index and the current index. Each element is processed once, giving O(n) time.

What is the best approach for Two Sum?

The hash map approach is the most efficient solution. Iterate through the array once while storing previously seen numbers in a hash table. For each value, check if its complement (target minus the current number) already exists in the map. This achieves O(n) time complexity with O(n) extra space.

What data structure is used in Two Sum?

The optimal solution relies on a hash table (hash map or dictionary). The structure stores previously seen numbers and allows constant‑time lookups to check whether the complement of the current number already exists.

What is the time complexity of Two Sum?

Time complexity depends on the approach used. The brute force solution checks every pair and runs in O(n^2) time with O(1) space. The optimal hash map approach processes the array once and performs constant‑time lookups, giving O(n) time and O(n) space.

Two Sum Python or Java solution approach?

Both Python and Java typically implement the optimal solution using a hash map. In Python, a dictionary stores number-to-index mappings. In Java, a HashMap performs the same role. The algorithm remains identical and runs in O(n) time.

Is Two Sum asked at Google, Amazon, or Meta?

Two Sum is one of the most common array and hash table interview questions and has appeared in interview preparation sets for companies like Amazon, Meta, and Google. It is often used as a warm‑up problem to test knowledge of hash maps and basic algorithmic thinking.

1. Two Sum - Practice Online

1. Two Sum

Easy57.8% Acceptance

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.

Example 1:

Input: nums = [2,7,11,15], target = 9
Output: [0,1]
Explanation: Because nums[0] + nums[1] == 9, we return [0, 1].

Example 2:

Input: nums = [3,2,4], target = 6
Output: [1,2]

Example 3:

Input: nums = [3,3], target = 6
Output: [0,1]

Constraints:

2 <= nums.length <= 10⁴
-10⁹ <= nums[i] <= 10⁹
-10⁹ <= target <= 10⁹
Only one valid answer exists.

Follow-up: Can you come up with an algorithm that is less than O(n²) time complexity?