Implement divide & conquer logic in JavaScript to implement QuickSort

We are required to write a JavaScript function that takes in an array of numbers and uses the quick sort algorithm to sort it.

QuickSort

This algorithm is basically a divide and conquer algorithm where we pick a pivot in every pass of loop and put all the elements smaller than pivot to its left and all greater than pivot to its right (if its ascending sort otherwise opposite)

Example

Let’s write the code for this function −

const arr = [43, 3, 34, 34, 23, 232, 3434, 4, 23, 2, 54, 6, 54];
// Find a "pivot" element in the array to compare all other
// elements against and then shift elements before or after
// pivot depending on their values
const quickSort = (arr, left = 0, right = arr.length - 1) => {
let len = arr.length, index;
if(len > 1) {
index = partition(arr, left, right)
if(left < index - 1) {
quickSort(arr, left, index - 1)
}
if(index < right) {
quickSort(arr, index, right)
}
}
return arr
}
const partition = (arr, left, right) => {
let middle = Math.floor((right + left) / 2),
pivot = arr[middle],
i = left, // Start pointer at the first item in the
array
j = right // Start pointer at the last item in the array
while(i <= j) {
// Move left pointer to the right until the value at the
// left is greater than the pivot value
while(arr[i] < pivot) {
i++
}
// Move right pointer to the left until the value at the
// right is less than the pivot value
while(arr[j] > pivot) {
j--
}
// If the left pointer is less than or equal to the
// right pointer, then swap values
if(i <= j) {
[arr[i], arr[j]] = [arr[j], arr[i]] // ES6 destructuring swap
i++
j--
}
}
return i
}
console.log(quickSort(arr));

Output

The output in the console −

[
2,   3,    4,  6, 23,
23,  34,   34, 43, 54,
54, 232, 3434
]

Updated on: 15-Sep-2020

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