Formatted question description: https://leetcode.ca/all/969.html

# 969. Pancake Sorting (Medium)

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]

Companies:
Square

Related Topics:
Array, Sort

## Solution 1.

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

// Time: O(N^2)
// Space: O(1)
class Solution {
public:
vector<int> pancakeSort(vector<int>& A) {
vector<int> ans;
for (int i = A.size(); i > 0; --i) {
int j = i - 1;
for (; j >= 0 && A[j] != i; --j);
reverse(A.begin(), A.begin() + j + 1);
ans.push_back(j + 1);
reverse(A.begin(), A.begin() + i);
ans.push_back(i);
}
return ans;
}
};


Java

class Solution {
public List<Integer> pancakeSort(int[] A) {
List<Integer> list = new ArrayList<Integer>();
int length = A.length;
for (int i = length; i >= 1; i--) {
if (A[i - 1] != i) {
int flipIndex = 0;
for (int j = length - 1; j >= 0; j--) {
if (A[j] == i) {
flipIndex = j;
break;
}
}
if (flipIndex > 0) {
pancakeFlip(A, flipIndex);
}
pancakeFlip(A, i - 1);
}
}
return list;
}

public void pancakeFlip(int[] array, int index) {
int left = 0, right = index;
while (left < right) {
int temp = array[left];
array[left] = array[right];
array[right] = temp;
left++;
right--;
}
}
}