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
  
    &num) {
		std::unordered_set
   
     a; std::unordered_set
    
      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; } };