Boomerang Numbers

Create a function that counts the "boomerangs" in an array where, given array x and index n, x[n] and x[n+2] are the same, but x[n+1] is different.

[1, 5, 1] -> 1
[1, 5, 1, 5, 1] -> 3

Examples

assert num_boomerangs([1, 5, 1, 5, 1]) == 3

Notes

• Boomerangs can overlap, and you must account for this.