[leetcode]Longest Consecutive Sequence @ Python

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 = -1
        for i in dict:
            if dict[i] == False:
                curr = i+1; lenright = 0
                while curr in dict:
                    lenright += 1; dict[curr] = True; curr += 1
                curr = i-1; lenleft = 0
                while curr in dict:
                    lenleft += 1; dict[curr] = True; curr -= 1
                maxLen = max(maxLen, lenleft+1+lenright)
        return maxLen