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(vector<int> &num) {
        set<int> s(num.begin(), num.end());
        auto it = s.begin();
        int c = 1;
        int l = 1;
        int prev = *it;
        for(++it; it!=s.end();it++){
            if(*it == prev+1){
                c++;
            }
            else{
                l = max(l, c);
                c = 1;
            }
            prev = *it;
        }
        return max(l,c);
    }
};