<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:
        if not nums or len(nums) == 0 :
            return 0
        head_map= {}
        tail_map= {}
        processed_map={}
        for n in nums :
            if n in processed_map:
                continue
            if n + 1 in head_map and n - 1 in tail_map :
                head_map[tail_map[n - 1]] = head_map[n + 1]
                tail_map[head_map[n + 1]] = tail_map[n - 1]
                del head_map[n + 1]
                del tail_map[n - 1]
            elif n + 1 in head_map :
                head_map[n] = head_map[n+1]
                tail_map[head_map[n + 1]] = n
                del head_map[n + 1]
            elif n - 1 in tail_map :
                tail_map[n] = tail_map[n - 1]
                head_map[tail_map[n - 1]] = n
                del tail_map[n - 1]
            else :
                head_map[n] = n
                tail_map[n] = n

            processed_map[n] = 1

        max_len = 0
        for key,val in head_map.items() :
            if val - key + 1 > max_len :
                max_len = val - key + 1
        return max_len
39850