LeetCode in Kotlin

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).

Solution

class Solution {
    fun pancakeSort(arr: IntArray): List<Int> {
        val result: MutableList<Int> = ArrayList()
        for (i in arr.size downTo 1) {
            var max = Int.MIN_VALUE
            var index = 0
            for (j in 0 until i) {
                if (max < arr[j]) {
                    index = j + 1
                    max = arr[j]
                }
            }
            result.add(index)
            reverse(arr, index - 1)
            result.add(i)
            reverse(arr, i - 1)
        }
        return result
    }

    private fun reverse(arr: IntArray, index: Int) {
        for (i in 0..(index - 1) / 2) {
            val temp = arr[i]
            arr[i] = arr[index - i]
            arr[index - i] = temp
        }
    }
}

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