leetcode - Longest Consecutive Sequence

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;    }};