# 969. Pancake Sorting (Medium)

https://leetcode.com/problems/pancake-sorting/

Given an array `A`, we can perform a pancake flip: We choose some positive integer `k <= A.length`, then reverse the order of the first k elements of `A`.  We want to perform zero or more pancake flips (doing them one after another in succession) to sort the array `A`.

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

Example 1:

```Input: [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: A = [3, 2, 4, 1]
After 1st flip (k=4): A = [1, 4, 2, 3]
After 2nd flip (k=2): A = [4, 1, 2, 3]
After 3rd flip (k=4): A = [3, 2, 1, 4]
After 4th flip (k=3): A = [1, 2, 3, 4], which is sorted.
```

Example 2:

```Input: [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.
```

Note:

1. `1 <= A.length <= 100`
2. `A[i]` is a permutation of `[1, 2, ..., A.length]`

## Solutions

``````class Solution {
public List<Integer> pancakeSort(int[] A) {
List<Integer> ans = new ArrayList();
int N = A.length;

Integer[] B = new Integer[N];
for (int i = 0; i < N; ++i)
B[i] = i+1;
Arrays.sort(B, (i, j) -> A[j-1] - A[i-1]);

for (int i: B) {
for (int f: ans)
if (i <= f)
i = f+1 - i;