 # Implement Quick Sort

## Problem Explanation

• Quick sort is an efficient sorting algorithm. It’s an in-place algorithm so it doesn’t take any auxilary space.
• First pick a random pivot point around which move all the smaller elements to it to the left of it and the bigger elements to the right of it.
• After getting the pivotIndex which is essentially the fixed position of that element, we find other pivotIndex by recusirvely calling this function.
• Quick sort’s worst case is O(n2) but that can be avoided if we pick random pivot point, so that way it’s big O is O(nlogn).
• It’s space complexity is O(logn).
• It’s an unstable algorithm.
• ## Solutions

Solution 1 (Click to Show/Hide)
``````//Swapping array elements via ES6 array destructuring
function swap(arr, x, y) {
[arr[x], arr[y]] = [arr[y], arr[x]];
}

//Pivot function returns the fixed pivot point
function pivot(arr, left = 0, right = arr.length - 1) {
let shift = left;
for (let i = left + 1; i <= right; i++) {
//Move all the small elements on the left side
if (arr[i] < arr[left]) swap(arr, i, ++shift);
}

//Finally swapping the last element with the left
swap(arr, left, shift);
return shift;
}

function quickSort(array, left = 0, right = array.length - 1) {
if (left < right) {
let pivotIndex = pivot(array, left, right);

//Recusrively calling the function to the left of the pivot and to the right of the pivot
quickSort(array, left, pivotIndex - 1);
quickSort(array, pivotIndex + 1, right);
}
return array;
}
``````

Solution 2 (Click to Show/Hide)
``````function quickSort(array) {
if (array.length === 0) {
return [];
} else {
const pivotValue = array;
// Sort elements into three piles
let lesser = [];
let equal = [];
let greater = [];
for (let e of array) {
if (e < pivotValue) {
lesser.push(e);
} else if (e > pivotValue) {
greater.push(e);
} else {
equal.push(e);
}
}
return [...quickSort(lesser), ...equal, ...quickSort(greater)];
}
}
``````
1 Like