I benchmarked it and found it to be at least twice as slow as the ternary implemenatation. It looks nice though.

It's a question of what's more idiomatic. See Avdi Grimm's master work: Confident Ruby

The description specifies: "Input will always be a non-negative integer."

Don't think this solution is correct: when n and m are both negative, this solution will give you the positive result, when 0 should be returned.

yup youre right, i forgot to delete that out. thanks!

The 3rd return line will never get hit.

relevant: https://xkcd.com/1445/

Now, what takes longer: getting the maximum value from an array, or using a ternary?

Did you actually read that post? They show that it hardly matters for 100,000 iterations, and we already know this only needs to handle 2 strings.

Test.assertEquals(validParentheses( "()))" ), false);

Should be added, otherwise this logic passes submission:

first == '(' && last == ')' && length is even