leetcode[128]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) {
        if(num.empty())return 0;
        unordered_set<int> all;
        unordered_set<int> visited;
        for(int i=0;i<num.size();i++)
           all.insert(num[i]);
        int max=0;
        for(int i=0;i<num.size();i++)
        {
            if(visited.find(num[i])!=visited.end())continue;
            int count=1;
            visited.insert(num[i]);
            int left=num[i];
            while(all.find(--left)!=all.end())
            {
                count++;
                visited.insert(left);
            }
            int right=num[i];
            while(all.find(++right)!=all.end())
            {
                count++;
                visited.insert(right);
            }
            max=max>count?max:count;
        }
        return max;
    }
};