Two Sum

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?

Solution

Use a hashmap to cache target - num, so if you find a number you can use, you can use it to find the answer.

class Solution:
    def twoSum(self, nums: List[int], target: int) -> List[int]:
        dt = {}
        for index, num in enumerate(nums):
            if num in dt:
                return [dt[num], index]
            dt[target - num] = index

Table of Contents

Two Sum

Solution