128. Longest Consecutive Sequence

比较简单，利用一下hashmap就可以了

class Solution(object):
    def longestConsecutive(self, nums):
        """
        :type nums: List[int]
        :rtype: int
        """
        #用hashmap
        h = {}
        for n in nums:
            h[n] = 1
        
        length = 0
        for key in h:
            if h[key] == 1:
                i = key + 1
                j = key - 1
                while i in h:
                    h[i] -= 1
                    i += 1
                while j in h:
                    h[j] -= 1
                    j -= 1
                length = max(i - 1 - j, length)
        
        return length