Python  ||     2 lines,  T/S: 97%/ 77%
\nclass Solution:\n    def findMaxK(self, nums: List[int]) -> int:\n        nums = set(nums)\n        return max(({x for x in nums if -x in nums}), default=-1)\
```\nclass Solution:\n    def findMaxK(self, nums: List[int]) -> int:\n        nums = set(nums)\n        return max(({x for x in nums if -x in nums}), default=-1)\n```\n[https://leetcode.com/problems/largest-positive-integer-that-exists-with-its-negative/submissions/1246950981/](https://leetcode.com/problems/largest-positive-integer-that-exists-with-its-negative/submissions/1246950981/)\n\nI could be wrong, but I think that time complexity is *O*(*N*) and space complexity is *O*(*N*), in which *N* ~ `len(set(nums))`.
['Python', 'Python3']