Leetcode Longest Consecutive Sequence

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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.

Analysis

We can use a hashset to record all the elements in the array. Then for each number in the array, we expand to both left and right and record the length of the expansion. 

See the following Java code: