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.

这是一道拿在手上不知道怎么下手的题目，题目tag提示是并查集，但是感觉和传统的并查集合并思路差别很大．参考了discussion里面的一个解决思路，主要是应用hashmap记录每个元素所属连续区间的长度，每次更新现有区间的两边节点的value为当前序列的长度．代码如下：

class Solution(object):
    def longestConsecutive(self, nums):
        """
        :type nums: List[int]
        :rtype: int
        """
        #how to use the union-find how to union
        #smallest,largest element 
        hash = {}
        res = 0
        for i in nums:
            if i not in hash:　＃防止重复
                left = hash[i-1] if i-1 in hash else 0　＃获取左边部分的长度
                right = hash[i+1] if i+1 in hash else 0　＃获取右边部分的长度
                total = left + right + 1
                hash[i] = total
                res = max(res, total)　　＃更新最长长度
                hash[i-left] = total　　　＃更新边缘点的长度，非常关键，方便之后的合并
                hash[i + right] = total　　＃更新边缘点的长度，非常关键，方便之后的合并
        return res