Pancake Sorting

arraytwo pointersgreedy

Problem Statement

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.

Examples

Example 1

Input: arr = [3,2,4,1]
Output: [4,2,4,3]
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: []
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 ).

🤔

Medium

Difficulty

Topic Info

ModuleDSA

CategoryArray

Sub-topicTwo Pointers