<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>

#include <vector>
#include <algorithm>

class Solution {
public:
    int longestConsecutive(std::vector<int>& nums) {
        if (nums.empty()) return 0;

        std::sort(nums.begin(), nums.end());
        
        // Remove duplicates
        nums.erase(std::unique(nums.begin(), nums.end()), nums.end());
        
        std::vector<int> diff;
        for (size_t i = 0; i < nums.size() - 1; ++i) {
            int m = nums[i + 1] - nums[i];
            diff.push_back(m);
        }

        int count = 1;
        std::vector<int> output;
        for (size_t i = 0; i < diff.size(); ++i) {
            if (diff[i] == 1) {
                count++;
            } else {
                output.push_back(count);
                count = 1;
            }
        }
        output.push_back(count);
        
        std::sort(output.begin(), output.end());
        return output.back();
    }
};

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