# 128-longest-consecutive-sequence


Try it on <a href='https://leetcode.com/problems/128-longest-consecutive-sequence'>leetcode</a>

## Description
<div class="description">
<div><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>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>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>
</div>
</div>

## Solution(Python)
```Python
class Solution:
    def longestConsecutive(self, nums):

        longest_streak = 0
        num_set = set(nums)

        for num in nums:
            if num-1 not in num_set:
                current_streak = 1
                cur_num = num
                while cur_num+1 in num_set:
                    cur_num += 1
                    current_streak += 1
                longest_streak = max(longest_streak,current_streak)

        return longest_streak
```