# Quick sort Algorithm

Quick sort like merge sort is a sorting algorithm under divide and conquer paradigm of algorithms like merge sort. Basic idea of algorithm is to divide inputs around a pivot and then sort two smaller parts recursively and finally get original input sorted.

### Selection of pivot

Entire idea of quick sort revolves around pivot. Pivot is an element in input around which input is arranged in such a way that all elements on left side are smaller and all elements on right side are greater than pivot. Question is how to find or select pivot and put it into correct position.

To make things simpler to start with, let’s assume first element of input is pivot element.

To put this pivot at correct position in input, start with next element of pivot in input space and find first element which is greater than pivot. Let that be ith position.

At the same time, start from end of array and find first element which is smaller than pivot. Let it be jth position.

If i and j have not crossed each other i.e i < j, then swap element at ith and jth positions, and continue moving right on input to find element greater than pivot and moving left to find element smaller than pivot.
Once i and j cross each other, swap pivot with element at jth position.  After this step, pivot will be at its correct position and array will be divided into two parts. All elements on left side will be less than pivot and all elements on right side will be greater than pivot.

### Quick sort partition example

This is too much to process, I know! Let’s take an example and see how it does it work? We have an array as follows Let’s select first element as pivot, pivot = 3. Start from next element of pivot, move towards right of array, till we see first element which is greater than pivot i.e. 3. From end of array, move towards left till you find an element which is less than pivot. Now, there are two indices, i and j, where A[i] > pivot and A[j] < pivot. See that i and j not yet crossed each other. Hence, we swap A[i] with A[j]. Array at the bottom of pic, shows resultant array after swap. Again, start with i+1 and follow the same rule : Stop when you find element greater than pivot. In this case, 10 is greater than 3, hence we stop. Similarly, move left from end again, till we find an element which is less than pivot. In this case, we end up at index = 2 which is element 1. Since, i > j, than means paths have been crossed. At this time, instead of swapping element at i and j index, swap element at j index with pivot. After swapping pivot with jth index, we have array divided into two parts, pivot as boundary. All elements on left side of pivot are smaller (they may not be sorted) and all elements on right side of pivot are greater than pivot (again may not be sorted). We, apply this same partition process to left and right arrays again, till base condition is hit. In this case, base condition would be if there is only one element in array to be partitioned.

### Quick sort algorithm

quickSort([], start, end)
1. If array has more than one elements i.e (start < end):
1.1 Find correct place for pivot.
pivot = partition(arr, low, high)
1.2. Apply same function recursively to left of pivot index
quickSort(arr, start, pivot -1 )
and to the right of pivot index
quickSort(arr, pivot + 1, end)

### Quick sort implementation

```package AlgorithmsAndMe;

public class QuickSort {

private void swap(int [] a, int i, int j){
int temp = a[i];
a[i] = a[j];
a[j] = temp;
}

private int partition(int [] a, int start, int end){
int pivot = a[start];
int i  = start;
int j  = end;

while(i < j){
while(i <= end && a[i] <= pivot) i++;
while(j >= start && a[j] > pivot) j--;

if(i < j) {
swap(a, i, j);
}
}

swap(a, start, j);
return j;
}

public void quickSort(int [] a, int start, int end){
if(start < end){
int p = partition(a, start, end);
quickSort(a,start, p-1);
quickSort(a, p+1, end);
}
}
}

```

There is another implementation which is based on Lomuto partition scheme, in this scheme, we make last element as pivot. The implementation is compact but complexity is bit higher than the original partition methods in terms of number of swaps.

```#include<stdlib.h>
#include<stdio.h>

void swap(int *a, int *b){
int temp = *a;
*a = *b;
*b = temp;
}

int partition(int a[], int low, int high)
{
// set pivot as highest element
int x  = a[high];

//Current low points to previous of low of this part of array.
int i = low - 1;

for (int j = low; j <= high-1; j++)
{
/*Move in the array till current node data is
less than the pivot */
if (a[j] <= x){
//set the current low appropriately
i++;
swap(&a[i], &a[j]);
}
}
//Now swap the next node of current low with pivot

swap(&a[i+1], &a[high]);

printf("\n Pivot : %d\n", a[i+1]);
for(int j=0; j<=high; j++){

printf("%d ", a[j]);
}
//return current low as partitioning point.
return i+1;
}

/* A recursive implementation of quicksort for linked list */
void quickSortUtil(int a[], int low, int high)
{
if (low < high)
{
int p = partition(a,low, high);
quickSortUtil(a,low, p-1);
quickSortUtil(a, p+1, high);
}
}

/* Driver program to run above code */
int main(){

int a[] = {5,4,2,7,9,1,6,10,8};

int size = sizeof(a)/sizeof(a);
quickSortUtil(a, 0, size-1);

for(int i=0; i<size; i++){
printf("%d ", a[i]);
}
return 0;
}
```

### Complexity analysis of quick sort algorithm

If pivot splits original array into two equal parts (which is the intention), complexity of quick sort is O(n log n). However, worst case complexity of quick sort happens when input array is already sorted in increasing or decreasing order. In this case, array is partitioned into two subarrays, one with size 1 and other with size n-1. Similarly, subarray with n-1 elements, it again is divided into two subarrays of size 1 and n-2. In order to completely sort array it will split for n-1 times and each time it requires to traverse n element to find correct position of pivot. Hence overall complexity of quick sort comes out as O(n2).

There is a very interesting question, which tests your understanding of system basics. Question is what is space complexity of this algorithm? There is no apparent memory is used. However, recursive implementation internally puts stack frames on stack for partitioned indices and function call return address and so on. In worst case, there can be n stack frames, hence worst case complexity of quick sort will be O(n).

How can we reduce that? If the partition with fewest elements is (recursively) sorted first, it requires at most O(log n) space. Then the other partition is sorted using tail recursion or iteration, which doesn’t add to the call stack. This idea, was described by R. Sedgewick, and keeps the stack depth bounded by O(log n) and hence space complexity will be O(log n).

## Quick sort with tail recursion

```Quicksort(A, p, r)
{
while (p < r)
{
q = Partition(A, p, r)
Quicksort(A, p, q)
p = q+1
}
}
```

Selection of Pivot
If array is completely sorted, then worst case behavior of quick sort is O(n2), so there comes another problem. How can we select pivot so that two subarrays are almost equal size. There are many solutions proposed.
1. Taking median of array as pivot. So how to select median of an unsorted array. We look into this problem separately, but yes it guarantees two halves of same size.
2. Selecting pivot randomly. This requires heuristics algorithms to select pivot.

Please leave your comment in case you find something wrong or you have some improved version.