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  1. PO
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    <p>Each iteration of the sort is suppose to reduce the sortable range. Basically, it's a divide and conquer algorithm.</p> <p>The <code>pivot</code> finds the "mid" point of the range you've specified</p> <p>The problem is, you're randomising the <code>pivot</code>, so each time you run through the sort, it's comparing some random section, probably including some section you've already sorted...</p> <p>So, instead of using...</p> <pre><code>int pivot = r.nextInt(high-low+1)+low; </code></pre> <p>You should be using...</p> <pre><code>int pivot = low + (high - low) / 2; </code></pre> <p>For example...</p> <pre><code>import java.util.ArrayList; import java.util.Collections; import java.util.List; import java.util.Random; public class QuickSort { static String[] strings; public static void main(String[] args) { List&lt;String&gt; values = new ArrayList&lt;&gt;(26); for (int index = 0; index &lt; 26; index++) { values.add(new String(new char[]{(char)(65 + index)})); } Collections.shuffle(values); strings = values.toArray(new String[values.size()]); System.out.println("Before"); for (int i = 0; i &lt; strings.length; i++) { System.out.print(strings[i] + " "); } System.out.println(""); qsort(0, strings.length - 1); System.out.println("The array, quicksorted: "); for (int i = 0; i &lt; strings.length; i++) { System.out.print(strings[i] + " "); } System.out.println("\n"); } static void qsort(int low, int high) { int i = low, j = high; // Get the pivot element int pivot = low + (high - low) / 2; String value = strings[pivot]; // Divide into two lists while (i &lt;= j) { while (strings[i].compareTo(value) &lt; 0) { i++; } while (strings[j].compareTo(value) &gt; 0) { j--; } if (i &lt;= j) { exchange(i, j); i++; j--; } } // Recursion if (low &lt; j) { qsort(low, j); } if (i &lt; high) { qsort(i, high); } } static void exchange(int i, int j) { String temp = strings[i]; strings[i] = strings[j]; strings[j] = temp; } } </code></pre> <p>Which produces...</p> <pre><code>Before N S B A F Z X J Q K V C L R W E H Y M U G I D P T O The array, quicksorted: A B C D E F G H I J K L M N O P Q R S T U V W X Y Z Before G U P Q D A W T R M E O X J S C I V Y F H L B N Z K The array, quicksorted: A B C D E F G H I J K L M N O P Q R S T U V W X Y Z Before D Z C B Q O K W X F V G R S A U P T H Y I E N L M J The array, quicksorted: A B C D E F G H I J K L M N O P Q R S T U V W X Y Z Before Q K H B W N J V C Y U O R P G I F D Z E L S A X M T The array, quicksorted: A B C D E F G H I J K L M N O P Q R S T U V W X Y Z Before R V P G E S C A H W X I T D Z B K Q F M U Y L J N O The array, quicksorted: A B C D E F G H I J K L M N O P Q R S T U V W X Y Z Before L O T E U D H N P J V I Q C X S Z W A R F K G Y B M The array, quicksorted: A B C D E F G H I J K L M N O P Q R S T U V W X Y Z Before I E J F U X P K R Q L S C O Y W G A Z B V M D H N T The array, quicksorted: A B C D E F G H I J K L M N O P Q R S T U V W X Y Z Before X L K T W E V J N Y G H O Q I M C P A R B F S U Z D The array, quicksorted: A B C D E F G H I J K L M N O P Q R S T U V W X Y Z Before U X N T K Q S V P F W C G Y O L A B E H J R D M Z I The array, quicksorted: A B C D E F G H I J K L M N O P Q R S T U V W X Y Z Before A J Z C M Y O Q F L K D P S X W N T I B H E R U V G The array, quicksorted: A B C D E F G H I J K L M N O P Q R S T U V W X Y Z </code></pre> <p>nb- Don't stress of the use of <code>List</code>, I just generated some random <code>String</code>s ;)</p>
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    1. POJava String Quicksort sporadically works?
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    1. USMadProgrammer
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    1. COI know. Trouble is I was specifically tasked with using a random pivot. My instructor said this would ensure the best average processing time. Since I changed from the code you shared to the random pivot, this has been happening.
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