Stuck in a simple quick sort implementation with random pivot

Question

I'm reviewing algorithm stuff and stuck in a simple quick sort algorithm implementation in java

import java.util.Arrays;
import java.util.Random;

public class QuickSort {
    int arr[];
    boolean randomPick;
    Random rand = null;

    public QuickSort(int[] inputArray) {
        arr = inputArray;
        randomPick = false;
    }

    public QuickSort(int[] inputArray, boolean random) {
        arr = inputArray;
        if (random) {
            randomPick = true;
            rand = new Random(System.currentTimeMillis());
        }
        else {
            randomPick = false;
        }
    }

    public int[] sort() {
        int start = 0;
        int end = arr.length - 1;
        try {
            _sort(start, end);
        }
        catch (StackOverflowError e) {
            System.out.println("StackOverflow: " + Arrays.toString(arr));
        }
        return arr;
    }

    private void _sort(int start, int end) {
        int i = start;
        int j = end;
        int pivotLoc;
        if (!randomPick) {
            pivotLoc = (j + i) / 2;
        }
        else {
            pivotLoc = rand.nextInt(j - i) + i;
        }
        int pivot = arr[pivotLoc];
        while (i < j) {   // swapping numbers around the pivot
            while (arr[i] < pivot) {
                i++;
            }
            while (arr[j] > pivot) {
                j--;
            }
            if (i < j) {
                int t = arr[i];
                arr[i] = arr[j];
                arr[j] = t;
                i++;
                j--;
            }
        }
        if (i - start > 1) {
            _sort(start, i);
        }
        if (end - i > 1) {
            _sort(i, end);
        }
    }
}

The code can either pick the middle number as the pivot or randomly pick a number as the pivot and it sorts the array by calling _sort recurrently. It works in the first case but fails in the second.

The situation is: when it reaches a subarray as this {3,5,5}, i==start==0 and j==end==2. Finally 5 and 5 are swapped, i becomes 2 and j becomes 1 (i++ and j--). Then since i - start>1 it will call _sort over and over and eventually evoke the stackoverflow error. However the error is supposed to happen in the first case (fixed pivot), which hasn't happened so far...

I don't know what's wrong with my code. I know it's not a pleasure to read code written by others but any help?

Answer 1

You've made a small mistake in your recursion condition, both the start and end compares are against i, the start should be against j

You have:

    if (i - start > 1) {
        _sort(start, i);
    }
    if (end - i > 1) {
        _sort(i, end);
    }

Needs to be:

    if (j > start) {
        _sort(start, j);
    }
    if (end > i) {
        _sort(i, end);
    }

And your i and j comparison needs to allow equals:

   // here ----v
        if (i <= j) {
            int t = arr[i];
            arr[i] = arr[j];
            arr[j] = t;
            i++;
            j--;
        }