Package g0901_1000.s0969_pancake_sorting

Class Solution

  • public class Solution
extends Object
    969 - Pancake Sorting.

    Medium

    Given an array of integers arr, sort the array by performing a series of pancake flips.

    In one pancake flip we do the following steps:

    • Choose an integer k where 1 <= k <= arr.length.
    • Reverse the sub-array arr[0...k-1] ( 0-indexed ).

    For example, if arr = [3,2,1,4] and we performed a pancake flip choosing k = 3, we reverse the sub-array [3,2,1], so arr = [1,2,3,4] after the pancake flip at k = 3.

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

    Example 1:

    Input: arr = [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: arr = [3, 2, 4, 1]

    After 1st flip (k = 4): arr = [1, 4, 2, 3]

    After 2nd flip (k = 2): arr = [4, 1, 2, 3]

    After 3rd flip (k = 4): arr = [3, 2, 1, 4]

    After 4th flip (k = 3): arr = [1, 2, 3, 4], which is sorted.

    Example 2:

    Input: arr = [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.

    Constraints:

    • 1 <= arr.length <= 100
    • 1 <= arr[i] <= arr.length
    • All integers in arr are unique (i.e. arr is a permutation of the integers from 1 to arr.length).

    • Constructor Detail

      • Solution

        public Solution()

    • Method Detail

      • pancakeSort

        public List<Integer> pancakeSort​(int[] arr)