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.
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 } }
CPP
/* C implementation QuickSort */ #include<stdio.h> // 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; } |
Sorted array: 1 5 7 8 9 10
Please refer complete article on QuickSort for more details!