**Problem:**

Implement a function **`maxIndex`** that takes in an array (or a vector) of numbers and returns the index of the highest number in the array. If the array is empty, the function should return **`-1`**.

Here is an example of how the function should work:

maxIndex([1, 2, 3]); // Returns 2
maxIndex([3, 2, 1]); // Returns 0
maxIndex([1, 3, 2, 3]); // Returns 1
maxIndex([]); // Returns -1