Is Sort Array Using Prefix Reversals easy or hard?

Sort Array Using Prefix Reversals is usually considered a medium-level problem. The implementation is straightforward once you identify the greedy strategy, but recognizing the correct sequence of flips can take practice.

Sort Array Using Prefix Reversals Python/Java solution

Python solutions typically use slicing or two-pointer reversal helpers, while Java implementations rely on manual swapping inside a reverse function. Both versions implement the same greedy pancake sort logic with O(n²) time complexity.

How to solve Sort Array Using Prefix Reversals in O(n)?

An O(n) solution is not generally achievable for arbitrary arrays using only prefix reversals because locating and repositioning elements requires repeated scans and flips. The accepted interview solution is the O(n²) greedy pancake sort algorithm.

What is the best approach for Sort Array Using Prefix Reversals?

The greedy pancake sort approach is the standard solution. It repeatedly places the largest unsorted element into its final position using at most two prefix reversals. The algorithm runs in O(n²) time with O(1) extra space.

Is Sort Array Using Prefix Reversals asked at Google/Amazon/Meta?

Prefix reversal and pancake sorting problems appear in coding interviews focused on greedy algorithms, array manipulation, and in-place operations. Variants of this problem have been discussed in interviews at large companies including Google, Amazon, and Meta.

What data structure is used in Sort Array Using Prefix Reversals?

The problem mainly uses arrays with in-place reversal operations. No advanced data structures are required, although careful index manipulation is necessary to perform efficient prefix flips.

What is the time complexity of Sort Array Using Prefix Reversals?

The optimal practical solution runs in O(n²) time because each iteration scans the unsorted section to locate the maximum element and performs prefix reversals. Space complexity stays O(1) since all operations are done in place.

3991. Sort Array Using Prefix Reversals - Practice Online

You are given an integer array nums of length n, where nums is a permutation of the integers in the range [0, n - 1].

You are also given an integer array pre, where each pre[i] is a valid prefix length.

In one operation, you may choose any length x from pre and reverse the first x elements of nums.

For example, applying a prefix reversal of length 3 on [4, 1, 2, 3] results in [2, 1, 4, 3].

Return the minimum number of operations required to sort nums in ascending order. If it is impossible to sort nums, return -1.

Example 1:

Input: nums = [2,0,1], pre = [2,3]

Output: 2

Explanation:

Reverse pre[1] = 3 elements to get nums = [1, 0, 2].
Then reverse pre[0] = 2 elements to get nums = [0, 1, 2].
Thus, the minimum number of prefix reversal required is 2.

Example 2:

Input: nums = [1,0,2], pre = [1,3]

Output: -1

Explanation:

It is impossible to sort the array using the given prefix lengths, so the answer is -1.

Example 3:

Input: nums = [0,1], pre = [2]

Output: 0

Explanation:

Since nums is already sorted, no prefix reversals are needed. Thus, the answer is 0.

Constraints:

1 <= n == nums.length <= 8
0 <= nums[i] <= n - 1
1 <= pre.length <= n
1 <= pre[i] <= n
nums is a permutation of integers from 0 to n - 1.
pre consists of unique integers.