A boomerang is a V-shaped sequence that is either upright or upside down. Specifically, a boomerang can be defined as: sub-array of length 3, with the first and last digits being the same and the middle digit being different.
Some boomerang examples:
[3, 7, 3], [1, -1, 1], [5, 6, 5]
Create a function that returns the total number of boomerangs in an array.
To illustrate:
[3, 7, 3, 2, 1, 5, 1, 2, 2, -2, 2]
// 3 boomerangs in this sequence: [3, 7, 3], [1, 5, 1], [2, -2, 2]
Be aware that boomerangs can overlap, like so:
[1, 7, 1, 7, 1, 7, 1]
// 5 boomerangs (from left to right): [1, 7, 1], [7, 1, 7], [1, 7, 1], [7, 1, 7], and [1, 7, 1]
countBoomerangs([9, 5, 9, 5, 1, 1, 1]) ➞ 2
countBoomerangs([5, 6, 6, 7, 6, 3, 9]) ➞ 1
countBoomerangs([4, 4, 4, 9, 9, 9, 9]) ➞ 0
[5, 5, 5] (triple identical digits) is NOT considered a boomerang because the middle digit is identical to the first and last.