给定一个未排序的整数数组，找出最长连续序列的长度。
要求算法的时间复杂度为 O(n)。

用set做 
class Solution:
    def longestConsecutive(self, nums: List[int]) -> int:
        if not nums oor len(nums)==0:
            return 0
        a =set(nums)
        res = 0

        for i in range(0,len(nums)):
            if a and nums[i] in a:
                a.remove(nums[i])
                low,high = nums[i],nums[i]
                low-=1
                while low in a:
                    a.remove(low)
                    low-=1
                
                high+=1
                while high in a:
                    a.remove(high)
                    high+=1
                res = max(res,high-low-1)

        return res