scratchcards

approach

each card is parsed into two sets of numbers, and the matches are their intersection.

part 1 scores a card as 1 << (matches - 1) when it has any matches. part 2 keeps a list with the number of copies of every card, starting at one each. walking the cards in order, a card with k matches adds its current copy count to each of the next k cards, and the answer is the sum of the list.

the trick

process cards in order and push each card's copy count forward instead of simulating every copy.

def part2(games):
    res = 0
    copies = [1 for _ in range(len(games))]
    for i, (winning, have) in enumerate(games):
        inter = have & winning
        for j in range(1, len(inter) + 1):
            copies[i + j] += copies[i]
    return sum(copies)