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);
                    list.add(flipIndex + 1);
                }
                pancakeFlip(A, i - 1);
                list.add(i);
            }
        }
        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--;
        }
    }
}

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