First clear solution for Card Game by juestr - python coding challenges

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First solution in Clear category for Card Game by juestr

def cards(deck, hand):
    last = -1
    for n in sorted(hand):
        if last == n:
            return False
        last = n - (n - last != 1)
    return last < deck

May 19, 2019

Comments:

oLeBeLo on Nov. 6, 2019, 3:54 p.m.
Wow! Short and clever solution. Would you be willing to share the logic behind it?

juestr on Nov. 6, 2019, 8:38 p.m.
Sure. I am going through the cards in sorted order, that is, each card (front) will be equal to or higher than the previous one. To be valid sometimes an equal card is allowed since the back sides can be n-1 and n+1 respective. But not if we have seen n-1 already, which due to the sorted order, must have been the previous one. Also in a series "1 2 2 3 5 5..." we know that one of the 2s has 3 (2+1) on the other side, so 3 needs to have 4 on the other side, and another 3 would thus be impossible. But anything goes for 5 again, since there is a gap between 3 and 5. As for the calculation of `last`, note that a bool in int context is just 0 or 1, so `last` is either the previous `n` or `n`-1 depending on the gap to the previous one. `last` signifies one of 2 states really, whether a repeat value is allowed or not. A new card with the value of `last` is proof of an invalid deck. The reason for the starting value `last = -1`: if and only if the first card is 0 we'll be in the no-gap, no repeat allowed state. The final check for the deck size also depends on the limit set by `last` rather than `n`, since `n` may or may not be equal to the deck size depending on the cards before.

oLeBeLo on Nov. 8, 2019, 5:34 a.m.
Ow! Didn't notice the power of working with the sorted values. I totally got it, thanks for your clear and detailed explanation, it's a beautiful solution. Thanks a bunch ;)

Escher

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