# 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.

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**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
```

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**Constraints:**

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