原题地址：https://oj.leetcode.com/problems/longest-consecutive-sequence/
题意：
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.
解题思路：使用一个哈希表，在Python中是字典dict数据类型。dict中的映射关系是{x in num：False}，这个表示num中的x元素没有被访问过，如果被访问过，则为True。如果x没有被访问过，检查x+1，x+2...，x-1，x-2是否在dict中，如果在dict中，就可以计数。最后可以求得最大长度。
代码：
class Solution:
# @param num, a list of integer
# @return an integer
def longestConsecutive(self, num):
dict = {x: False for x in num} # False means not visited
maxLen = -
for i in dict:
if dict[i] == False:
curr = i+; lenright =
while curr in dict:
lenright += ; dict[curr] = True; curr +=
curr = i-; lenleft =
while curr in dict:
lenleft += ; dict[curr] = True; curr -=
maxLen = max(maxLen, lenleft++lenright)
return maxLen