Quick Sort not working as expected in java

Question

I learned the theory about QuickSort. I tried to implement this algorithm in java but I am not seeing all the numbers are being sorted.I am not able to point out what mistake I am making.

QuickSort.java

public class QuickSort {

    public void quicksort(int[] arr, int lb, int ub) {
        //up=upper bound
        //lb=lower bound
        
        if(lb<ub) {
          int loc=partition(arr,lb,ub);
          quicksort(arr,lb,loc-1);
          quicksort(arr,loc+1,ub);
        }
        
    }
    
    public int partition(int[] arr,int lb,int ub) {
        int pivot=arr[lb];
        int start=lb;
        int end=ub-1;
        
        while(start<end) {
            while(arr[start]<=pivot && start<arr.length-1) {
                start++;
                }
        
        while(arr[end]>pivot) {
            end--;
        }
        
        if(start<end) {
            swap(start,end,arr);
        }
        }
        
        swap(lb,end,arr); //swapping pivot value with end
        return end;
    }
    
      public void swap(int start,int end,int[] arr) {
         // System.out.println(end+" "+pivot);
          int temp=arr[start];
          arr[start]=arr[end];
          arr[end]=temp;
      }

}

Main class:

import java.util.Arrays;

public class Program {
    
    public static void main(String[] args) {
        int[] arr= {7,6,10,5,9,2,1,15,7};
        QuickSort q=new QuickSort();
      q.quicksort(arr,0,arr.length);
      System.out.println(Arrays.toString(arr));
    }

}

My output coming is:

[2, 5, 6, 7, 1, 7, 9, 10, 15]

I rechecked my code and done some debugging but for first pass it is working fine.I dont know,at what point it is being wrong?

Answer 1

This dude made exactly what you are looking for :) take a look at his partition function here:

Random pivot quicksort in Java

Here is what you can do for the helper functions:

public static void sort(int arr[], int lb, int ub) {
    if (lb < ub) {

        int part = partition(arr, lb, ub);

        // Sorts the partitions
        sort(arr, lb, part - 1);
        sort(arr, part + 1, ub);
    }
}

public static void displayArray(int[] arr) {
    System.out.print("[ ");
    for (int x :
            arr) {
        System.out.print(x + " ");
    }
    System.out.print("]");
}

public static void main(String[] args) {

    int[] arr = {7, 6, 10, 5, 9, 2, 1, 15, 7};
    int len = arr.length;

    displayArray(arr);
    sort(arr, 0, len-1);
    System.out.println();
    displayArray(arr);

}