Java Data Structures - Quick Sort

Quick sort is a highly efficient sorting algorithm and is based on partitioning of array of data into smaller arrays. A large array is partitioned into two arrays one of which holds values smaller than the specified value, say pivot, based on which the partition is made and another array holds values greater than the pivot value.

Quick sort partitions an array and then calls itself recursively twice to sort the two resulting subarrays. This algorithm is quite efficient for large-sized data sets as its average and worst case complexity are of Ο(n2), where n is the number of items.

Algorithm

Based on our understanding of partitioning in quick sort, we will now try to write an algorithm for it, which is as follows −

Step 1: Choose the highest index value has pivot.
Step 2: Take two variables to point left and right of the list excluding pivot.
Step 3: left points to the low index.
Step 4: right points to the high.
Step 5: while value at left is less than pivot move right.
Step 6: while value at right is greater than pivot move left.
Step 7: if both step 5 and step 6 does not match swap left and right.
Step 8: if left ≥ right, the point where they met is new pivot.

Quick Sort Algorithm

Using pivot algorithm recursively, we end up with smaller possible partitions. Each partition is then processed for quick sort. We define recursive algorithm for quicksort as follows −

Step 1: Make the right-most index value pivot.
Step 2: partition the array using pivot value.
Step 3: quicksort left partition recursively.
Step 4: quicksort right partition recursively.

Example

import java.util.Arrays;

public class QuickSortExample {
   int[] intArray = {4,6,3,2,1,9,7};
   
   void swap(int num1, int num2) {
      int temp = intArray[num1];
      intArray[num1] = intArray[num2];
      intArray[num2] = temp;
   }
   int partition(int left, int right, int pivot) {
      int leftPointer = left -1;
      int rightPointer = right;

      while(true) {
         while(intArray[++leftPointer] < pivot) {
            // do nothing
         }
         while(rightPointer > 0 && intArray[--rightPointer] > pivot) {
            // do nothing
         }
         
         if(leftPointer >= rightPointer) {
            break;
         } else {
            swap(leftPointer,rightPointer);
         }
      }
      swap(leftPointer,right);
      
      // System.out.println("Updated Array: "); 
      return leftPointer;
   }
   void quickSort(int left, int right) {
      if(right-left <= 0) {
         return;   
      } else {
         int pivot = intArray[right];
         int partitionPoint = partition(left, right, pivot);
         quickSort(left,partitionPoint-1);
         quickSort(partitionPoint+1,right);
      }        
   }
   public static void main(String[] args) { 
      QuickSortExample sort = new QuickSortExample();
      int max = sort.intArray.length;
      System.out.println("Contents of the array :");
      System.out.println(Arrays.toString(sort.intArray));
      
      sort.quickSort(0, max-1);
      System.out.println("Contents of the array after sorting :");
      System.out.println(Arrays.toString(sort.intArray));
   }
}

Output

Contents of the array :
[4, 6, 3, 2, 1, 9, 7]
Contents of the array after sorting :
[1, 2, 3, 4, 6, 7, 9]