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Find the longest gap

A binary gap within a positive number num is any sequence of consecutive zeros that is surrounded by ones at both ends in the binary representation of num.

For example:

9 has binary representation 1001 and contains a binary gap of length 2.

529 has binary representation 1000010001 and contains two binary gaps: one of length 4 and one of length 3.

20 has binary representation 10100 and contains one binary gap of length 1.

15 has binary representation 1111 and has 0 binary gaps.

Write function gap(num) that, given a positive num, returns the length of its longest binary gap.

The function should return 0 if num doesn’t contain a binary gap.

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