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

Constraints:

- `0 <= nums.length <= 105`
- `-109 <= nums[i] <= 109`