Two Sum (LC 1)

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.

class Solution:
    def twoSum(self, nums: List[int], target: int) -> List[int]:
        num_map = {}  # value -> index
 
        for i, n in enumerate(nums):
            diff = target - n
            if diff in num_map:
                return [num_map[diff], i]
            num_map[n] = i

• The idea is to create a hashmap (value -> index). We can just start with an empty map instead of creating a whole map of the array.
• For a given number n in the array, we calculate its difference from the target.
• If this difference is in our map, we can return the index of the difference and our current number.
• If it’s not, we will add n and its index i to the hashmap as the key and value respectively.

Uglier hashmap solution that initializes:

class Solution:
    def twoSum(self, nums: List[int], target: int) -> List[int]:
        
        num_map = {}  # val -> index
 
        for i, n in enumerate(nums):
            num_map[n] = i
 
        for i, n in enumerate(nums):
            diff = target - n
            
            if diff in num_map and num_map[diff] != i:
                return [num_map[diff], i]