Add lonely number problem (#37)

algorithms/array.md

@@ -56,3 +56,5 @@ Arrays
 - Given an array, return the length of the longest increasing contiguous subarray.
   - E.g., `[1, 3, 2, 3, 4, 8, 7, 9]`, should return `4` because the longest increasing array is `[2, 3, 4, 8]`.
   - [Source](http://blog.gainlo.co/index.php/2017/02/02/uber-interview-questions-longest-increasing-subarray/).
+- Given an array of integers where every value appears twice except one, find the single, non-repeating value. Do so with a running time of O(1).
+  - E.g., `[2, 5, 3, 2, 1, 3, 4, 5, 1]` returns 4, because it is the only value that appears in the array only once.