Updated array.md with a Problem

I just wanted to add a problem that I got from an interview.

algorithms/array.md

@@ -4,6 +4,9 @@ Arrays
 #### Hard
 - Given a set of rectangles represented by a height and an interval along the y-axis, determine the size of its union. ([Solution](https://www.geeksforgeeks.org/divide-and-conquer-set-7-the-skyline-problem/))
 - Given an array, find the longest arithmetic progression. ([Solution](https://www.geeksforgeeks.org/longest-arithmetic-progression-dp-35/))
+- Given an unsorted array of integers, find the length of the longest consecutive elements sequence. 
+- E.g.,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. ([Solution](https://www.geeksforgeeks.org/longest-consecutive-subsequence/))
+    Note: Your algorithm should run in O(n) complexity.
 
 #### Medium
 - Given a list of item prices, find all possible combinations of items that sum a particular value `K`. ([Solution](https://www.geeksforgeeks.org/combinational-sum/))