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.
Example 1:
Input: nums = [100, 4, 200, 1, 3, 2]
Output: 4
Explanation: The longest consecutive elements sequence is [1, 2, 3, 4]. Therefore its length is 4.
Example 2:
Input: nums = [0, 3, 7, 2, 5, 8, 4, 6, 0, 1]
Output: 9
Example 3:
Input: nums = [1, 0, 1, 2]
Output: 3
Constraints:
0 <= nums.length <= 105-109 <= nums[i] <= 109
How do you know if a number is the absolute beginning of a sequence?