longest consecutive sequence

# average tc: O(n), worst case tc: O(n*n) - because of hash collisions it can happen
# that the worst case of hash look up is O(n)

class Solution:
    def longestConsecutive(self, nums: List[int]) -> int:

        hashset = set()
        longest_streak = 0

        for num in nums:
            hashset.add(num)

        for i in range(len(nums)):
            # check if that it is the greatest number in neighbouring
            if nums[i] + 1 not in hashset:
                current_streak = 1
                current_num = nums[i]
                # keep checking untill you reach an endpoint
                while current_num-1 in hashset:
                    current_streak += 1
                    current_num -= 1

                longest_streak = max(longest_streak, current_streak)

        return longest_streak