Two Sum

Description

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: nums[0] + nums[1] = 2 + 7 = 9, so we return [0, 1].

Example 2:
Input: nums = [3, 2, 4], target = 6
Output: [1, 2]
Explanation: nums[1] + nums[2] = 2 + 4 = 6, so we return [1, 2].

Example 3:
Input: nums = [3, 3], target = 6
Output: [0, 1]
Explanation: nums[0] + nums[1] = 3 + 3 = 6, so we return [0, 1].

Constraints

2 <= nums.length <= 10^4
-10^9 <= nums[i] <= 10^9
-10^9 <= target <= 10^9
Only one valid answer exists

Signature

def solution(nums: list[int], target: int) -> list[int]: ...

Complexity

Time: O(n)
Space: O(n)

Hints

Pattern: Hash map for complement lookup
Approach: For each number x, check if (target - x) exists in hash map. If yes, return indices. Otherwise, add x to map.
Complexity: Use hash map to store seen values and their indices

Solutions

Python

def solution(nums, target):
    container = {}

    for i in range(len(nums)):
        if nums[i] in container:
            return [container[nums[i]], i]
        t = target - nums[i]
        container[t] = i

    return []