128\. Longest Consecutive Sequence

Medium

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:**

*   <code>0 <= nums.length <= 10<sup>5</sup></code>
*   <code>-10<sup>9</sup> <= nums[i] <= 10<sup>9</sup></code>