You know exactly who you're looking for

You have ₹9 of change to spend at a canteen and want two items that cost exactly that. When you pick up a ₹3 samosa, you don't need to search: the only partner that works costs exactly ₹6.

So walk the prices once. At each one, the partner you need is the target minus this price. Ask one question: have I already seen that partner?

The partner: the target minus this value. Just look it up.

Target 9target = 9

Need 6. Not seen yet. Store 3 → 0.

Move 1 of 3

Remember where you saw each value

The answer is a pair of positions, so the map stores each value with the box you saw it in. At each box, look up the partner first. If it's there, you have your two positions. If not, store this value and its box, so a later value can find it.

Look up before you store. Otherwise a 3 with a target of 6 could pair with itself.

Look up first: then store, or a value pairs with itself.

In code

where = {}
for j, x in enumerate(nums):
    need = target - x
    if need in where:
        return [where[need], j]
    where[x] = j