TwoSum (Hashtable Way)

Question

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:

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

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

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

Solution

Note: When given the array is sorted, it's might be a better idea to use TwoSum (Two Pointers way)

The idea is as you go through the array, you use a dictionary to record what you have

class Solution:
    def twoSum(self, arr: List[int], target: int) -> List[int]:
        numberIndex = {}  # {number: index}

        for index, num in enumerate(arr):
            remainder = target - num

            if remainder in numberIndex:
                return [index, numberIndex[remainder]]

            numberIndex[num] = index
        return []

Time complexity: $O(n)$
Space complexity: $O(logn)$