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.
0 <= nums.length <= 10^5-10^9 <= nums[i] <= 10^9nums = [100, 4, 200, 1, 3, 2]4nums = [0, 3, 7, 2, 5, 8, 4, 6, 0, 1]9nums = [1, 0, 1, 2]3