# C++ Program for 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.

1. Always pick first element as pivot.
2. Always pick last element as pivot (implemented below)
3. Pick a random element as pivot.
4. 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.
Pseudo Code for recursive QuickSort function :

```/* low  --> Starting index,  high  --> Ending index */
quickSort(arr[], low, high)
{
if (low < high)
{
/* pi is partitioning index, arr[p] is now
at right place */
pi = partition(arr, low, high);

quickSort(arr, low, pi - 1);  // Before pi
quickSort(arr, pi + 1, high); // After pi
}
}
```

 `/* C implementation QuickSort */` `#include ` ` `  `// A utility function to swap two elements ` `void` `swap(``int``* a, ``int``* b) ` `{ ` `    ``int` `t = *a; ` `    ``*a = *b; ` `    ``*b = t; ` `} ` ` `  `/* 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 ` ` `  `    ``for` `(``int` `j = low; j <= high- 1; j++) ` `    ``{ ` `        ``// If current element is smaller than or ` `        ``// equal to 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++) ` `        ``printf``(``"%d "``, arr[i]); ` `    ``printf``(``"n"``); ` `} ` ` `  `// Driver program to test above functions ` `int` `main() ` `{ ` `    ``int` `arr[] = {10, 7, 8, 9, 1, 5}; ` `    ``int` `n = ``sizeof``(arr)/``sizeof``(arr[0]); ` `    ``quickSort(arr, 0, n-1); ` `    ``printf``(``"Sorted array: n"``); ` `    ``printArray(arr, n); ` `    ``return` `0; ` `} `

Please refer complete article on QuickSort for more details!

GeeksforGeeks has prepared a complete interview preparation course with premium videos, theory, practice problems, TA support and many more features. Please refer Placement 100 for details

My Personal Notes arrow_drop_up
Article Tags :
Practice Tags :

6

Please write to us at contribute@geeksforgeeks.org to report any issue with the above content.