Create a function which returns the indices where the monotonicity of a 1-D list changes. If there are none, return an empty list. A monotonic list is one that is either non-increasing or non-decreasing.
Examples
group_monotonic([0, 1]) ➞ []
group_monotonic([0, 2, 1]) ➞ [1]
group_monotonic([0, 1, 1, 0]) ➞ [2]
Notes
- Trivially, all points and line-segments are monotonic (see example #1).
- Return the indices where each monotonic section stops, not where each new one begins: i.e. return the "peaks" of the triangles (see example #2).
- Monotonic lists are allowed to be constant (see example #3).
- You can expect positive and negative values in the list.