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`

Solution:

Time complexity: O(N)

Space complexity: O(1)