Write a function that returns an array of elements [number, multiplicity]. The multiplicity of a number refers to the number of times it occurs in the array.
To illustrate:
[5, 5, 1, 1, 5, 5, 3]
[[5, 4], [1, 2], [3, 1]]
// Since 5 appears 4 times, 1 appears twice, and 3 appears once.
The final array should only include unique elements, and the elements should be ordered by when they first appeared in the original array.
multiplicity([1, 1, 1, 2, 2, 3]) ➞ [[1, 3], [2, 2], [3, 1]]
multiplicity([1, 1, 1, 1, 1]) ➞ [[1, 5]]
multiplicity([1, 5, 4, 3, 2]) ➞ [[1, 1], [5, 1], [4, 1], [3, 1], [2, 1]]
N/A