Quicksort is a beautiful algorithm. It's considered the fastest sorting algorithm in practice. Most implementations I've seen online are complicated. This post will demonstrate a simple implementation that trades space complexity for readability.

## How does quicksort work?

3 steps.

**Choose**a pivot element.**Partition**: Put all elements smaller than the pivot in a smaller array, (say, smaller_subarray). Put all elements greater than the pivot in another array (say, greater_subarray).**Recurse & Merge**: Recursively sort the smaller and greater sub-arrays. Merge the sorted arrays with the pivot.

## How do we implement it in Python?

Let's start with the high-level function `quicksort()`

```
def quicksort(array: List[int]) -> List[int]:
"""Recursive implementation of quicksort.
Args:
array (List[int]): a list of integers
Returns:
List[int]: sorted array
"""
# Base case
if len(array) <= 1:
return array
# Step 1: choose a pivot
pivot_idx = random.choice(range(len(array)))
# Step 2: partition
smaller_subarray, greater_subarray = partition(array, pivot_idx)
# Step 3: Recurse on smaller and greater subarrays.
return (
quicksort(smaller_subarray)
+ [array[pivot_idx]]
+ quicksort(greater_subarray)
)
```

Let's go line by line,

### Base case

```
if len(array) <= 1:
return array
```

Every recursive function must have one. If we have an empty or single-item array, there's nothing to sort. We simply return the array.

### Step 1: Choose a pivot

`pivot_idx = random.choice(range(len(array)))`

The choice of pivot is the difference between having an `O(N log N)`

vs `O(N^2)`

time complexity.

How so? Let's take an example.

The worst-case involves having an already sorted array.

`array = [1,2,3,4,5]`

The image below shows recursion trees using two different pivot selection strategies:

**Legend**

- Blue lines => the greater subarray.
- Orange lines => the smaller subarray.
- Red lines => the pivot.

Left shows the case where we always choose the first/last item as the pivot. This leads to higher recursion depth as the subproblem size reduces by 1 at each level. We have roughly

`O(N)`

levels / recursive calls. Since joining the sub-arrays with the pivot is`O(N)`

, this leads to`O(N^2)`

time complexity.Right shows the ideal case where we choose the median as the pivot at every level. This leads to balanced sub-problems. The recursion depth is lower, roughly

`O(log N)`

. This leads to`O(N log N)`

time complexity.

Choosing a random pivot leads to an *expected* runtime of `O(N log N)`

time complexity. We use that in this implementation.

Wait! There's still one case where `quicksort()`

could be `O(N^2)`

, even after choosing a randomized/median pivot. This is the case when every item in the array is identical. We could add a check to avoid this case as well.

According to this source ,

The probability that quicksort will use a quadratic number of compares when sorting a large array on your computer is much less than the probability that your computer will be struck by lightning!

### Step 2: Partition

We write the high-level function assuming that we have a magic function `partition()`

which returns two arrays.

`smaller_subarray`

: all elements`<=`

the pivot`greater_subarray`

: all elements`>`

the pivot

The implementation of `partition()`

is quite simple. We compare each item in the array with the pivot. If it's `<=`

pivot, we add it to the smaller sub-array, else we add it to the greater sub-array.

```
def partition(array: List[int], pivot_idx: int) -> Tuple[List[int], List[int]]:
"""Parition array into subarrays smaller and greater than the pivot.
Args:
array (List[int]): input array
pivot_idx (int): index of the pivot
Returns:
Tuple[List[int], List[int]]: smaller subarray, greater subarray
"""
smaller_subarray, greater_subarray = [], []
for idx, item in enumerate(array):
# we don't want to add pivot to any of the sub-arrays
if idx == pivot_idx:
continue
if item <= array[pivot_idx]:
smaller_subarray.append(item)
else:
greater_subarray.append(item)
return smaller_subarray, greater_subarray
```

### Step 3:

```
return (
quicksort(smaller_subarray)
+ [array[pivot_idx]]
+ quicksort(greater_subarray)
)
```

This step is quite exquisite. We recurse on the smaller and greater sub-arrays and place the pivot in between them. The beauty of a recursive implementation is that we could re-use our function with a smaller size of the input and trust that it would work.

Let's unroll it a bit.

`quicksort(smaller_subarray)`

=> returns a sorted version of the`smaller_subarray`

`[array[pivot_idx]]`

=> out pivot in a list. It's needed for concatenating it with the other two lists.`quicksort(greater_subarray)`

=> returns a sorted version of the`greater_subarray`

Now, we join the 3 lists including the pivot, and return the final sorted list.

## Complexity

### Time

**Expected** run-time of `O(N log N)`

since it's a randomized implementation.

We do `O(N)`

work at each `quicksort()`

call. The major components are `partition()`

and merge step - both are `O(N)`

.

For randomized implementation, we have O(log N) levels/recursive calls.

So, expected time complexity ~ `Number of recursive calls`

* `work done per recursive call`

~ `O(N) * O(log N)`

~ `O(N log N)`

### Space

`O(N)`

since we use extra space for storing the smaller and greater sub-arrays. The recursion stack also uses `O(log N)`

space. Overall, this implementation uses `O(N)`

space.

The in-place implementation without using auxiliary lists would lead to an `O(log N)`

space complexity.

## The code

The implementation including the `partition()`

and tests are here.