Two Sums

The Two Sum question is a leetcode question that works on finding two numbers in an array whose sum equals a given target

Let’s understand the question:

Given an array nums and a target value, return the indices of the two numbers such that they add up to the target.

example

target = 9

Output : [2,7]
because 2+ 7 -> 9

*Approach *

I came to know there are 3 ways to solve this question
they were

*brute force - > comparing all possible pairs
hashing -> store all previous traverse
optimized -> by difference *

I choose optimized approach it is the most time and memory efficent approach, I started with a question "Do I really need to check all pairs?”
That felt slow and so was hashing “Can I remember what I’ve already seen?”

step 1: Target - current number
and move that value to another list
dif = []

loop 1

nums = [2, 7, 11, 15] 
dif = []
9 - nums[0] - > 2

2 not in dif 
so -> dif = [2]

loop 2

nums = [2, 7, 11, 15] 
9-nums[1] -> 2

2 in dif 

then that means the pair is found

Algorithm

def twoSum(self, nums, target):
        seen = {}  # number : index

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