QuickSort

Like Merge Sort, QuickSort is a Divide and Conquer algorithm. It picks an element as pivot and partitions the given array around the picked pivot. There are many different versions of quickSort that pick pivot in different ways. 

Always pick first element as pivot.
Always pick last element as pivot (implemented below)
Pick a random element as pivot.
Pick median as pivot.
The key process in quickSort is partition(). Target of partitions is, given an array and an element x of array as pivot, put x at its correct position in sorted array and put all smaller elements (smaller than x) before x, and put all greater elements (greater than x) after x. All this should be done in linear time.

/* C++ implementation of QuickSort */
#include <bits/stdc++.h>
using namespace std;

// A utility function to swap two elements
void swap(int* a, int* b)
{
	int t = *a;
	*a = *b;
	*b = t;
}
this is quick sort 
/* This function takes last element as pivot, places
the pivot element at its correct position in sorted
array, and places all smaller (smaller than pivot)
to left of pivot and all greater elements to right
of pivot */
int partition (int arr[], int low, int high)
{
	int pivot = arr[high]; // pivot
	int i = (low - 1); // Index of smaller element and indicates the right position of pivot found so far

	for (int j = low; j <= high - 1; j++)
	{
		// If current element is smaller than the pivot
		if (arr[j] < pivot)
		{
			i++; // increment index of smaller element
			swap(&arr[i], &arr[j]);
		}
	}
	swap(&arr[i + 1], &arr[high]);
	return (i + 1);
}

/* The main function that implements QuickSort
arr[] --> Array to be sorted,
low --> Starting index,
high --> Ending index */
void quickSort(int arr[], int low, int high)
{
	if (low < high)
	{
		/* pi is partitioning index, arr[p] is now
		at right place */
		int pi = partition(arr, low, high);

		// Separately sort elements before
		// partition and after partition
		quickSort(arr, low, pi - 1);
		quickSort(arr, pi + 1, high);
	}
}

/* Function to print an array */
void printArray(int arr[], int size)
{
	int i;
	for (i = 0; i < size; i++)
		cout << arr[i] << " ";
	cout << endl;
}

// Driver Code
int main()
{
	int arr[] = {10, 7, 8, 9, 1, 5};
	int n = sizeof(arr) / sizeof(arr[0]);
	quickSort(arr, 0, n - 1);
	cout << "Sorted array: \n";
	printArray(arr, n);
	return 0;
}

// This code is contributed by rathbhupendra