Given an unsorted array of integers *nums*, return *the length of the longest consecutive elements sequence*. You must write an algorithm that runs in O(n) time.
nums = [array of integers]
integer (length of longest consecutive sequence)
Example 1:
Input:
nums = [100,4,200,1,3,2]
Output:
4
Explanation:
The longest consecutive sequence is [1,2,3,4], so output 4.
Example 2:
Input:
nums = []
Output:
0
Explanation:
Empty array has longest sequence length 0.