1. Two Sum.md

1. Two Sum (Easy)

[tag] hash table, two pointers

• use a hash table, key: num, value: index.
• need to first look up on the dict then add number to avoid case of 4 + 4 = 8.

class Solution:
    def twoSum(self, nums: List[int], target: int) -> List[int]:
        num_dict = defaultdict(int)
        ans = []
        for i in range(len(nums)):
            nums_to_find = target - nums[i]
            if nums_to_find in num_dict:
                return [i, num_dict[nums_to_find]]                
            else:
                num_dict[nums[i]] = i
        return [-1, -1]