Find Unique Number in Array

Create a function that takes an array of integers as an argument and returns a unique number from that array. All numbers except unique ones have the same number of occurrences in the array.

Examples

findSingleNumber([2, 2, 2, 3, 4, 4, 4]) ➞ 3

findSingleNumber([2]) ➞ 2

findSingleNumber([]) ➞ null

findSingleNumber([7, 13, 3, 6, 5, 4, 4, 13, 5, 3, 6, 7, 6, 5, 3, 13, 4, 7, 13, 5, 7, 4, 3, 6, 8, 4, 3, 7, 5, 6, 13]) ➞ 8

findSingleNumber([1, 2, 3, 6, 5, 4, 4, 2, 5, 3, 6, 1, 6, 5, 3, 2, 4, 1, 2, 5, 1, 4, 3, 6, 101, 4, 3, 1, 5, 6, 2]) ➞ 101

Notes

• Be aware this challenge includes two validations:
  1. Empty input should return null (example #3).
  2. Single item arrays should return that item (example #2).
• There are always 1 or 0 unique numbers in the input. No two or three+ uniques.