Two-sum: find two products whose prices add up to a budget

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# Q: Find two products whose prices add up exactly to a budget (two-sum).
#
# O(n) time and O(n) memory, instead of O(n²) with two loops. For money I work
# with integer cents.
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def two_sum(prices: list[int], budget: int) -> tuple[int, int] | None:
    seen = {}                                # price -> index
    for i, p in enumerate(prices):
        if budget - p in seen:
            return seen[budget - p], i
        seen[p] = i
    return None