The problem states that the array has every integer from 1 to n exactly once except for some integer in between, which appears twice. The array that you're testing on doesn't fit this description (it has a 9 but no 6, for example), so the solution doesn't work.

I tried this with: arr = [1, 5, 7, 3, 2, 9, 2, 8]
And it returned 9 instead of 2 :/

Clever

O(n²) instead of O(n).

gotcha.

...because that would have been an incorrect formula.

Test failed with month = 1: 0 should equal 1
Test failed with month = 7: 2 should equal 3
Test Passed
Test Passed
Test Passed
Test failed with month = 4: 1 should equal 2
Test failed with month = 2: 0 should equal 1
Test failed with month = 10: 3 should equal 4
Test failed with month = 11: 3 should equal 4
Test failed with month = 8: 2 should equal 3
Test Passed
Test failed with month = 5: 1 should equal 2

this is a reference to the instance of Number on which you called the method times.

Who is "this" ?

Hardly ever?

Not really sure, but I'd say there is undefined behavior. Some compilers may prefer to eval LHS first and increment p before testing the character. See, e.g., https://en.cppreference.com/w/c/language/eval_order