1. Two Sum

class Solution(object):
    def twoSum(self, nums, target):
        """
        :type nums: List[int]
        :type target: int
        :rtype: List[int]
        """
        seen = {} # use dictionary data structure
        for i in range(len(nums)): # i is the position/key of the num
            num2 = target - nums[i]  # num2 is the value
            if num2 in seen: # num2 is the key
                return [seen[num2], i]
            else:
                seen[nums[i]] = i
        return ‘Not found‘