Pancake Sorting — LeetCode 969 Python Solution

The problem

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.

Example

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.

Python solution

class Solution:
    def pancakeSort(self, arr: List[int]) -> List[int]:
        def reverse(arr, j):
            i = 0
            while i < j:
                arr[i], arr[j] = arr[j], arr[i]
                i, j = i + 1, j - 1

        n = len(arr)
        ans = []
        for i in range(n - 1, 0, -1):
            j = i
            while j > 0 and arr[j] != i + 1:
                j -= 1
            if j < i:
                if j > 0:
                    ans.append(j + 1)
                    reverse(arr, j)
                ans.append(i + 1)
                reverse(arr, i)
        return ans

Complexity

Pattern: Two Pointers

Use the order already in the input to discard half the search space at every step. LeetCode 969. Pancake Sorting is filed here because LeetCode tags it Two Pointers, which is the vocabulary this hub collects.

The two pointers guide has the Python template for the pattern and the 201 LeetCode problems that use it.

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