Given an unsorted array of integers, find the length of the longest consecutive elements sequence. Well, lets assume that this task related only to the sequence with step equal to 1 only.
Given exampleArray = [100, 4, 200, 1, 3, 2].
The longest consecutive elements sequence is [1, 2, 3, 4].
longestConsecutiveLength(exampleArray) returns the length of the longest sequence - 4.
Your algorithm should have O(n) complexity.