Given an unsorted array of integers, find the length of the longest consecutive elements sequence.

For example,
Given [100, 4, 200, 1, 3, 2],
The longest consecutive elements sequence is [1, 2, 3, 4]. Return its length: 4.

Your algorithm should run in O(n) complexity.

class Solution {
public:
    int longestConsecutive(std::vector<int> &num) {
		std::unordered_set<int> a;
		std::unordered_set<int> b;
		for (int i = 0; i < num.size(); i++)
		{
			a.insert(num[i]);
		}
		int max = 0;
		for (int i = 0; i < num.size(); i++)
		{
			if(b.find(num[i]) != b.end()) continue;
			int cnt = 1,n = num[i],m = num[i];
			while(a.find(--n) != a.end()) b.insert(n),cnt++;
			while(a.find(++m) != a.end()) b.insert(m),cnt++;
			max = max > cnt ? max : cnt;
		}
		return max;
    }
};

leetcode - Longest Consecutive Sequence