class Solution(object):
    def twoSum(self, nums, target):
        """
        :type nums: List[int]
        :type target: int
        :rtype: List[int]
        """
        # diff contains (key, value) pairs
        # key - the difference between target number and the current number
        # value - the index of the number
        diff = {}
        for index in range(len(nums)):
            if nums[index] in diff:
                return [diff[nums[index]], index]

            # to avoid duplicate keys problem
            # though the problem has restricted it

            #elif target - nums[index] in diff:
                #continue

            else:
                diff[target - nums[index]] = index

    return None