Given an unsorted array of integers, find the length of the longest consecutive elements sequence. Your algorithm should run in O(n) complexity.
Input: [100, 4, 200, 1, 3, 2] Output: 4
Explanation: The longest consecutive elements sequence is [1, 2, 3, 4]. Therefore its length is 4.
See the full details of the problem Longest Consecutive Sequence at LeetCode