There are no random tests.

ϕ = 
  n11 * n00 - n10 * n01
  ----------------
  sqrt√(n1 * n0 * n1 * n0)

This formula is wrong; the denominator should be sqrt((n00 + n01) * (n10 + n11) * (n00 + n10) * (n01 + n11)).

This problem has been copied from Eloquent Javascript which is licensed under Creative Commons attribution-noncommercial license.

In which it is clearly mentioned:

You are free to:

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    Adapt — remix, transform, and build upon the material

    The licensor cannot revoke these freedoms as long as you follow the license terms.


Under the following terms:

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So you've to give the credit in the description.