Longest Consecutive Sequence

""" Why O(n) ?
Since each element will be visited only twice, once in for loop and once in the while loop of its root (i.e if seq. is 2 3 4 then root of all of them is 2)
Thus asymptotically linear.
"""

class Solution:
    def longestConsecutive(self, nums: List[int]) -> int:
        lookup = set(nums)
        ans = 0

        for e in nums:
            if e-1 in lookup:
                continue

            cnt = 0
            while e in lookup:
                cnt += 1
                e += 1

            ans = max(ans, cnt)

        return ans