Question

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]

Difficulty:Medium

Category:Array, Sort

Analyze

Solution

class Solution {
 public:
  vector<int> pancakeSort(vector<int>& A) {
    vector<int> ans;
    for (int i = 0; i < A.size(); ++i) {
      auto maxelement_p = max_element(A.begin(), A.end() - i);
      ans.emplace_back(maxelement_p - A.begin() + 1);
      ans.emplace_back(A.size() - i);
      reverse(A.begin(), maxelement_p + 1);
      reverse(A.begin(), A.end() - i);
    }
    return ans;
  }
};