Given an array of integers `arr`, sort the array by performing a series of pancake flips.

In one pancake flip we do the following steps:

- Choose an integer `k` where `1 <= k <= arr.length`.
- Reverse the sub-array `arr[0...k-1]` (0-indexed).

For example, if `arr = [3,2,1,4]` and we performed a pancake flip choosing `k = 3`, we reverse the sub-array `[3,2,1]`, so `arr = [1,2,3,4]` after the pancake flip at `k = 3`.

Return an array of the `k`-values corresponding to a sequence of pancake flips that sort `arr`. Any valid answer that sorts the array within `10 * arr.length` flips will be judged as correct.

Example 1:

    Input: arr = [3,2,4,1]
    Output: [4,2,4,3]
    Explanation:
    We perform 4 pancake flips, with k values 4, 2, 4, and 3.
    Starting state: arr = [3, 2, 4, 1]
    After 1st flip (k = 4): arr = [1, 4, 2, 3]
    After 2nd flip (k = 2): arr = [4, 1, 2, 3]
    After 3rd flip (k = 4): arr = [3, 2, 1, 4]
    After 4th flip (k = 3): arr = [1, 2, 3, 4], which is sorted.

Example 2:

    Input: arr = [1,2,3]
    Output: []
    Explanation: The input is already sorted, so there is no need to flip anything.
    Note that other answers, such as [3, 3], would also be accepted.

Constraints:

- `1 <= arr.length <= 100`
- `1 <= arr[i] <= arr.length`
- All integers in `arr` are unique (i.e. `arr` is a permutation of the integers from `1` to `arr.length`).