"""

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.

 

Example 1:

Input: nums = [2,7,11,15], target = 9
Output: [0,1]
Output: Because nums[0] + nums[1] == 9, we return [0, 1].
Example 2:

Input: nums = [3,2,4], target = 6
Output: [1,2]
Example 3:

Input: nums = [3,3], target = 6
Output: [0,1]
 

Constraints:

2 <= nums.length <= 103
-109 <= nums[i] <= 109
-109 <= target <= 109
Only one valid answer exists.

"""

class Solution(object):
    def twoSum(self, nums, target):
        """
        :type nums: List[int]
        :type target: int
        :rtype: List[int]
        """
        
        # create an empty dictionary
        d = {}
        
        # iterate the key and value of the dic
        for i, num in enumerate(nums):
            # to check if the n value is already in the dictionary
            n = target - num
            # if not then record the num's key into the dictionary
            if n not in d:
                d[num] = i
                
            # if there is a n, then the key is d[n] and the i(at the moment)
            else:
                return [d[n],i]
