<p>Given an unsorted array of integers <code>nums</code>, return <em>the length of the longest consecutive elements sequence.</em></p>

<p>You must write an algorithm that runs in&nbsp;<code>O(n)</code>&nbsp;time.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>

<pre>
<strong>Input:</strong> nums = [100,4,200,1,3,2]
<strong>Output:</strong> 4
<strong>Explanation:</strong> The longest consecutive elements sequence is <code>[1, 2, 3, 4]</code>. Therefore its length is 4.
</pre>

<p><strong class="example">Example 2:</strong></p>

<pre>
<strong>Input:</strong> nums = [0,3,7,2,5,8,4,6,0,1]
<strong>Output:</strong> 9
</pre>

<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li><code>0 &lt;= nums.length &lt;= 10<sup>5</sup></code></li>
	<li><code>-10<sup>9</sup> &lt;= nums[i] &lt;= 10<sup>9</sup></code></li>
</ul>

class Solution:
    def longestConsecutive(self, nums: List[int]) -> int:
        lookup = {}
        res = 0
        for num in nums:
            if num in lookup:
                continue
            lookup[num] = 0
            left, right = num - 1, num + 1
            if left in lookup and right in lookup:
                sp = left - lookup[left]
                ep = right + lookup[right]
            elif left in lookup:
                sp = left - lookup[left]
                ep = num
            elif right in lookup:
                sp = num
                ep = right + lookup[right]
            else:
                sp = ep = num
            length = ep - sp
            lookup[sp] = lookup[ep] = length
            res = max(res, length + 1)
        return res
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