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]

Hints

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;
            ans.add(i);
            ans.add(N--);
        }

        return ans;
    }
}
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