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.

题意：给定一个未排序的数组，问最长的连续的数值有多少个。

题解：unordered_set内部用hash实现，均摊复杂度为O(1)，那么我们可以遍历数组中的这些数，将其向左向右延伸，并且在延伸查找的同时进行删除，减少重复遍历的次数。

class Solution {public:    //13ms    int longestConsecutive(vector<int>& nums) {        unordered_set<int>s(nums.begin(),nums.end());        int res = 0;        for (auto val : nums){            if (!s.count(val))  continue;            int pre = --val,next = ++val;            while (s.count(pre))    s.erase(pre--);            while (s.count(next))   s.erase(next++);            res = max(res,next - pre - 1);        }        return res;    }};