Two Sum Problem

https://leetcode.com/problems/two-sum/

Solution 1 - brute force
Solution 2 - hash table
- Maintain a dictionary of num: index
- Calculate the complement of the current number
- Find the complement in the dictionary
Solution 3 - with two pointer.

n = len(nums)
num_map = {}
for i in range(n):
    complement = target - nums[i]
    if complement in num_map:
        return (num_map[complement])
    num_map[nums[i] = i]
return None