diff --git a/Sorts/PancakeSort.js b/Sorts/PancakeSort.js new file mode 100644 index 0000000000..30596c3e85 --- /dev/null +++ b/Sorts/PancakeSort.js @@ -0,0 +1,79 @@ +/** + * Unlike a traditional sorting algorithm, which attempts to sort with the fewest + * comparisons possible, the goal of pancake sort is to sort the sequence in as few reversals as + * possible. The idea is to do something similar to Selection Sort. We one by one place + * maximum element at the end and reduce the size of current array by one. + * + * Source: https://www.geeksforgeeks.org/pancake-sorting/ + * + * This sorting algorithm is inspired by the pancake problem (hence the name), + * where a spatula can be placed anywhere between two pancakes and flip all pancakes + * above. + * + * The interesting about this algorithm (besides its name) is that instead of comparisons, + * the algorithm relies on flipping an array. + * + * Source: https://en.wikipedia.org/wiki/Pancake_sorting#The_original_pancake_problem + * + */ + +/** + * Unlike Array.prototype.reverse, flipArray reverses only a subarray of the given + * array, determined by the parameters startIndex and endIndex + * + * @param {number[]} array The array to flip + * @param {number} startIndex The start of the subarray + * @param {number} endIndex The end of the subarray + * @returns The flipped array + */ +export function flipArray(array, startIndex, endIndex) { + while (startIndex < endIndex) { + // swap front and back of the subarray + const temp = array[startIndex] + array[startIndex] = array[endIndex] + array[endIndex] = temp + + // essentialy reducing the problem to a smaller subarray + startIndex++ + endIndex-- + } + + return array +} + +/** + * Returns the index of the maximum number of a subarray in a given array + * + * @param {number[]} array The array to found the maximum number's index + * @param {*} startIndex The start of the subarray + * @param {*} endIndex The end of the subarray + * @returns The index of the maximum number + */ +export function findMax(array, startIndex, endIndex) { + let maxIndex = 0 + for (let i = startIndex; i <= endIndex; i++) { + if (array[i] > array[maxIndex]) maxIndex = i + } + + return maxIndex +} + +/** + * The Pancake Sort algorithm. Note that even though it's a completely different + * concept of sorting an array, it's rather simple! + * + * @param {number[]} array The array to sort + * @returns The sorted array + */ +export function pancakeSort(array) { + for (let subarraySize = array.length; subarraySize > 1; subarraySize--) { + const maximumIndex = findMax(array, 0, subarraySize - 1) + + if (maximumIndex !== subarraySize - 1) { + flipArray(array, 0, maximumIndex) + flipArray(array, 0, subarraySize - 1) + } + } + + return array +} diff --git a/Sorts/test/PancakeSort.test.js b/Sorts/test/PancakeSort.test.js new file mode 100644 index 0000000000..b9b3368bee --- /dev/null +++ b/Sorts/test/PancakeSort.test.js @@ -0,0 +1,24 @@ +import { flipArray, findMax, pancakeSort } from '../PancakeSort' + +describe('flipArray', () => { + it('should flip any subarray of any array', () => { + expect(flipArray([1, 2, 3, 4], 0, 3)).toEqual([4, 3, 2, 1]) + expect(flipArray([1, 2, 3, 4, 5], 2, 4)).toEqual([1, 2, 5, 4, 3]) + expect(flipArray([], 0, 0)).toEqual([]) + }) +}) + +describe('findMax', () => { + it('should find the index of the maximum value in any subarray of any array', () => { + expect(findMax([1, 3, 2, 5], 0, 3)).toEqual(3) + expect(findMax([1, 3, 2, 5], 0, 2)).toEqual(1) + }) +}) + +describe('pancakeSort', () => { + it('should sort any array', () => { + expect(pancakeSort([4, 3, 2, 1])).toEqual([1, 2, 3, 4]) + expect(pancakeSort([3, 1, 4, 2])).toEqual([1, 2, 3, 4]) + expect(pancakeSort([100, 1000, 10, 1])).toEqual([1, 10, 100, 1000]) + }) +})