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.

Solution: Update solution. This solution is from Peking2 (http://blog.sina.com.cn/s/blog_b9285de20101iqar.html).
This solution is much easier to understand.

class Solution {
public:
    int longestConsecutive(vector<int> &num) {
        unordered_set<int> s;
        int res = 0;
        for(int i = 0; i < num.size(); i++) {
            s.insert(num[i]);
        }
        for(int i = 0; i < num.size() && !s.empty(); i++) {
            int lower = num[i], upper = num[i];
            if(s.find(num[i]) == s.end()) continue;
            else s.erase(num[i]);
            while(s.find(upper+1) != s.end()) {
                s.erase(++upper);
            }
            while(s.find(lower-1) != s.end()) {
                s.erase(--lower);
            }
            res = max(res, upper-lower+1);
        }
        return res;
    }
};