面试题40：数组中最长的连续序列的长度

　　这道题是一个比较简单的题目，由于题目中要求O(n)，限制使用排序，所有借用hash表unordered_set<int> s来存储数组中元素，保证O(n)的时间复杂度。

题目：

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) {
        unordered_set<int> s(num.begin(),num.end());

        int res = 0;
        for(int n : num){
            if(s.find(n) != s.end()){
                s.erase(n);
                int pre = n - 1;
                int post = n + 1;
                while(s.find(pre) != s.end()){
                    s.erase(pre--);
                }
                while(s.find(post) != s.end()){
                    s.erase(post++);
                }
                res = max(res,post-pre-1);
            }
        }
        return res;
    }
};