Problem: Longest Consecutive Sequences
Problem Description: 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.

Not able to come out a solution within 15 min, find approach from http://blog.csdn.net/zxzxy1988/article/details/8589131