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Given an unsorted array of integers nums, return the length of the longest consecutive elements sequence. You must write an algorithm that runs in O(n) time.
Input: nums = [100,4,200,1,3,2]
Output: 4
Explanation: The longest consecutive sequence is [1,2,3,4].
Topics: arrays, hash-map
Asked by: Amazon, Google, Meta, Microsoft
Time complexity: O(n). Space complexity: O(n).