## Challenge Statement

- 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. - This challenge corresponds to LeetCode #128.

### Constraints

`0 <= nums.length <= 10`

^{5}`-10`

^{9}<= nums[i] <= 10^{9}

**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
```

## Solution

Below is my solution and some test cases. This solution has a **linear time complexity O(n) and a linear space complexity O(n)**, where n is the length of the input list*.*