It seems to be lacking a test case where there are positives and negatives, and a negative is the answer. Every test seems to have only positives, only negatives, or if they're mixed, the answer is always positive.
Eg. (7 -2 14 -7 22)
Correct answer here would be -2. You could have code that returns 7 or -7 because they're equal and you were testing for that to make it positive 7, and this would be wrong but you aren't marked down for it.
I was surprised anybody thought about the case for two negatives and it got me. I was being dumb. I think this might catch somebody now and then too.
Edit: Also, the hardest part of this was figuring out how to get at the numbers in the string, which I don't think was the intention. Compared to that, the rest was easy.