I know I'm 2 years late, but his code is about ~3 faster. Tested it with 1 and 10 million executions.

Approved by Johan

you are mutating the input array, this is why the assertion messages are confusing. i patched the tests so that the message is computed before the function call. your code is incorrect though, you need to fix it. also note that mutating your input when you are not asked to is bad practice, and in this kata in particular it cannot work as sometimes you will need to add an element.