JavaScript Quicksort recursive

We are required to write a JavaScript function that takes in an array of Numbers. The function should apply the algorithm of quicksort to sort the array either in increasing or decreasing order.

QuickSort Algorithm

Quicksort follows the below steps −

Step 1 − Make any element as the pivot (preferably first or last, but any element can be the pivot)

Step 2 − Partition the array on the basis of pivot

Step 3 − Apply a quick sort on the left partition recursively

Step 4 − Apply a quick sort on the right partition recursively

The average and best case time complexity of QuickSort are O(nlogn) whereas in worst cases, it can slow up to O(n^2).

Example

The code for this will be −

const arr = [5,3,7,6,2,9];
const swap = (arr, leftIndex, rightIndex) => {
   let temp = arr[leftIndex];
   arr[leftIndex] = arr[rightIndex];
   arr[rightIndex] = temp;
};
const partition = (arr, left, right) => {
   let pivot = arr[Math.floor((right + left) / 2)];
   let i = left;
   let j = right;
   while (i <= j) {
      while (arr[i] < pivot) {
         i++;
      };
      while (arr[j] > pivot) {
         j--;
      };
      if (i <= j) {
         swap(arr, i, j); //sawpping two elements
         i++;
         j--;
      };
   };
   return i;
}
const quickSort = (arr, left = 0, right = arr.length - 1) => {
   let index;
   if (arr.length > 1) {
      index = partition(arr, left, right);
      if (left < index - 1) {
         quickSort(arr, left, index - 1);
      };
      if (index < right) {
         quickSort(arr, index, right);
      };
   }
   return arr;
}
let sortedArray = quickSort(arr);
console.log(sortedArray);

Output

And the output in the console will be −

[ 2, 3, 5, 6, 7, 9 ]