Python-сообщество

a = [ 1,  2,  3,  4,  5,
      6,  7,  8,  9, 10,
     11, 12, 13, 14, 15,
     16, 17, 18, 19, 20]
b = [1, 6, 11, 16, 17, 12, 7, 2, 3, 8, 13, 18, 19, 14, 9, 4, 5, 10, 15, 20]

class SNAKE():
	def __init__(self, width, height):
		self.width = width
		self.height = height
		self.size = width*height
	
	def up(self, currentPosition):
		ret = currentPosition-self.width
		return self.size+ret if ret<0 else ret
	
	def down(self, currentPosition):
		return (currentPosition+self.width)%self.size
	def left(self, currentPosition):
		return (currentPosition//self.width)*self.width+(currentPosition-1)%self.width
	def right(self, currentPosition):
		return (currentPosition//self.width)*self.width+(currentPosition+1)%self.width
snake = SNAKE(5, 4)
cur = 0
S = []
while cur!=snake.size-1:
	S.append(cur)
	cur = snake.right(cur) if (cur//snake.width==snake.height-1 and (cur%snake.width)%2==0) or (cur//snake.width==0 and (cur%snake.width)%2==1) else \
		snake.up(cur) if (cur%snake.width)%2==1 else \
		snake.down(cur)
S.append(cur)
print(S)

cur = snake.right(cur) if (cur//snake.width==snake.height-1 and (cur%snake.width)%2==0) or (cur//snake.width==0 and (cur%snake.width)%2==1) else \
		snake.up(cur) if (cur%snake.width)%2==1 else \
		snake.down(cur)

if (cur//snake.width==snake.height-1 and (cur%snake.width)%2==0) or (cur//snake.width==0 and (cur%snake.width)%2==1):
	cur = snake.right(cur)
elif (cur%snake.width)%2==1:
	cur = snake.up(cur)
else:
	cur = snake.down(cur)

def idx_gen( sx, sy ):
    even_r = range( sy )
    odd_r = range( sy-1, -1, -1)
    for x in range( sx ):
        for y in (odd_r if x % 2 else even_r):
            yield x,y
a = range(1,21)
sx, sy = 5, 4
b = [ a[y*sx + x] for x, y in idx_gen(sx, sy) ]
print b

from itertools import chain,islice
l = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20]
dim = 5
print(
    list(
        chain.from_iterable(
            map(lambda idx: islice(l,idx,None,dim) if idx%2==0 else reversed(list(islice(l,idx,None,dim))), range(dim))
)))

from itertools import cycle
l = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20]
cols = 5
matrix = zip(*[iter(l)]*cols)
get_column = lambda index: [x[index] for x in matrix]
columns_orders = [lambda x: x, lambda x: x[-1::-1]]
get_column_order = cycle(columns_orders)
for col in xrange(cols):
	column = get_column(col)
	column = next(get_column_order)(column)
	print column

matrix = zip(*[iter(l)]*cols)