view CONTRIBUTING @ 32612:a8262b7784f9

py3: replace None with -1 to sort an integer array

In Python 2:

>>> ls = [4, 2, None]
>>> sorted(ls)
[None, 2, 4]

In Python 3:

>>> ls = [4, 2, None]
>>> sorted(ls)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: unorderable types: NoneType() < int()

Therefore we replaced None with -1, and the safe part is that, None and -1 are
only the keys which are used for sorting so we don't need to convert the -1's
back to None.

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