<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>

const int MAXN = 2e9 + 1;
const int MINV = -1e9;
class Solution {
public:
    int longestConsecutive(vector<int>& nums) {
        for (const auto &n : nums) {
            b.write(n);
        }
        int ans = 0;
        for (const auto &n : nums) {
            if (b.read(n)) {
                int t = 1;
                for (int cur = n - 1; cur >= MINV && b.read(cur); cur--) {
                    b.erase(cur);
                    t++;
                }
                b.erase(n);
                for (int cur = n + 1; cur <= -MINV && b.read(cur); cur++) {
                    b.erase(cur);
                    t++;
                }
                ans = std::max(ans, t);
            }
        }
        return ans;
    }
private:
    struct Bitmap {
        inline std::pair<int, char> norm(int v) {
            v -= MINV;
            fflush(stdout);
            return std::make_pair<int, char>(v / 8, v % 8);
        }
        inline void write(int v) {
            auto [vx, vb] = norm(v);
            bitmap[vx] |= 1<<vb;
        }
        inline void erase(int v) {
            auto [vx, vb] = norm(v);
            bitmap[vx] &= ~(char)((1<<vb));
        }
        inline bool read(int v) {
            auto [vx, vb] = norm(v);
            return bitmap[vx] & (1 << vb);
        }
        unsigned char bitmap[(MAXN+1)/8+1];
    };
    static Bitmap b;
};

Solution::Bitmap Solution::b;
16164