C++ Program for Quick Sort

Last Updated : 18 Oct, 2024

Quick Sort is one of the most efficient sorting algorithms available to sort the given dataset. It is known for its efficiency in handling large datasets which made it a go-to choice for programmers looking to optimize their code.

In C++, the STL’s sort() function uses a mix of different sorting algorithms, including QuickSort, but we can’t choose which one it uses. For a pure Quick Sort implementation, you can use the C function qsort(), but it might not work with STL containers. The best approach is to manually implement it.

What is Quick Sort Algorithm?

QuickSort is a comparison-based sorting algorithm that uses the divide-and-conquer approach to sort an array or list of elements. It can be implemented using two functions:

partition(): It is the key process in the quicksort algorithm. It involves selecting a pivot element and rearranging the array so that all elements smaller than the pivot are placed to its left, and the elements greater than the pivot are placed to its right. The point where the pivot is placed is called the partitioning index and it is returned to its caller quickSort().
quickSort(): It is the main recursive function that implements the divide-and-conquer strategy. It divides the given array into two subarrays based on the partitioning index returned by partition() function. Then it keeps calling itself for these two subarrays until the whole array is sorted.

Quick sort is a highly efficient sorting algorithm. The C++ Course provides step-by-step instructions on implementing quick sort, allowing you to enhance your sorting skills.

C++ Program to Implement Quick Sort

The below program sorts a vector using quicksort algorithm:

C++

// C++ Program to demonstrate how to implement the quick // sort algorithm #include <bits/stdc++.h> using namespace std;  int partition(vector<int> &vec, int low, int high) {      // Selecting last element as the pivot     int pivot = vec[high];      // Index of elemment just before the last element     // It is used for swapping     int i = (low - 1);      for (int j = low; j <= high - 1; j++) {          // If current element is smaller than or         // equal to pivot         if (vec[j] <= pivot) {             i++;             swap(vec[i], vec[j]);         }     }      // Put pivot to its position     swap(vec[i + 1], vec[high]);      // Return the point of partition     return (i + 1); }  void quickSort(vector<int> &vec, int low, int high) {      // Base case: This part will be executed till the starting     // index low is lesser than the ending index high     if (low < high) {          // pi is Partitioning Index, arr[p] is now at         // right place         int pi = partition(vec, low, high);          // Separately sort elements before and after the         // Partition Index pi         quickSort(vec, low, pi - 1);         quickSort(vec, pi + 1, high);     } }  int main() {     vector<int> vec = {10, 7, 8, 9, 1, 5};     int n = vec.size(); 	   	// Calling quicksort for the vector vec     quickSort(vec, 0, n - 1);      for (auto i : vec) {         cout << i << " ";     }     return 0; }

Output

1 5 7 8 9 10

Complexity Analysis of Quick Sort

Time Complexity: O(n log n) on average and O(n²) in worst case
Auxiliary Space: O(log n) on average and O(n) in the worst case.

Java Program for QuickSort

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C++ Program for Quick Sort

What is Quick Sort Algorithm?

C++ Program to Implement Quick Sort

Complexity Analysis of Quick Sort

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