LeetCode Q128: Longest Consecutive Sequence (hard)

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.

Solution:

Because requiring only take O(n), so considering using Hash table. For each number, search backward and forward.