f=lambda m,a=-1:m and m.pop(0)[::a]+f(m,-a)
print(f([]))
print(f([[1]]))
print(f([[2, 1], [3, 4]]))
print(f([[3, 2, 1], [4, 5, 6], [9, 8, 7]]))
print(f([[4, 3, 2, 1], [5, 6, 7, 8], [12, 11, 10, 9], [13, 14, 15, 16]]))
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[]
[1]
[1, 2, 3, 4]
[1, 2, 3, 4, 5, 6, 7, 8, 9]
[1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16]