Q2: Riffle Shuffle
The familiar riffle shuffle of a deck of cards (or in our case, of a sequence of things) results in a new configuration of cards in which the top card is followed by the middle card, then by the second card, then the card after the middle, and so forth. Assuming the deck (sequence) contains an even number of cards, write a list comprehension that produces the shuffled sequence.
Hint: To write this as a single comprehension, you may find the expression k%2, which evaluates to 0 on even numbers and 1 on odd numbers, to be useful. Consider how you can use the 0 or 1 returned by k%2 to alternatively access the beginning and the middle of the list.
分析：
我是直接根据样例二，找出规律对应的表达式，然后直接实现
def riffle(deck):
"""Produces a single, perfect riffle shuffle of DECK, consisting of
DECK[0], DECK[M], DECK[1], DECK[M+1], ... where M is position of the
second half of the deck. Assume that len(DECK) is even.
>>> riffle([3, 4, 5, 6])
[3, 5, 4, 6]
>>> riffle(range(20))
[0, 10, 1, 11, 2, 12, 3, 13, 4, 14, 5, 15, 6, 16, 7, 17, 8, 18, 9, 19]
"""
"*** YOUR CODE HERE ***"
#return [] if len(deck)==0 else deck[0:len(deck)//2]+deck[len(deck)-1:len(deck)//2-1:-1] if len(deck)%2==0 else deck[0:len(deck)//2]+deck[len(deck)-1:len(deck)//2-1:-1]
#return [] if len(deck)==0 else [deck[x] for x in range(0,len(deck)//2)]+[deck[x] for x in range(len(deck)-1,len(deck)//2-1,-1)]
return [deck[(i%2)*len(deck)//2+i//2]for i in range(len(deck))]
