Two-sum with a hash map

Given a list of numbers and a target, find the two whose values add up to that target. The obvious approach checks every pair - but that is slow, and there is a much better way that only walks the list once.

The trick is to trade a little memory for speed. As you move through the list, keep a dict of every value you have already seen and where it was. For each new number, the partner it needs is target - number. If that partner is already in your dict, you are done - you have both indices. If not, record the current number and move on. Each value gets looked up in constant time, so the whole search is a single pass.

Your task: write two_sum(nums, target) that returns the two indices whose values add up to target (in any order), or None if no such pair exists. Assume at most one valid pair.

You'll practice:

Using a dict as a lookup table you build as you go
Turning a slow all-pairs check into a single-pass scan

# Return the two indices whose values add up to target (in any order),
# or None if no such pair exists.
def two_sum(nums, target):
    pass