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POSelection algorithm returns wrong result sometimes
 

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  1. POSelection algorithm returns wrong result sometimes
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    <p>For a homework problem, I need to write a Java method to find the kth smallest number in an array, using quicksort-style partitioning. I was given the partition() method, and I'm supposed to write the method to get the kth smallest number.</p> <p>The problem asks that the pivot always be the rightmost element in the array range.</p> <p>I was given the following:</p> <pre><code>public int partition(int left, int right, long pivot) { int leftPtr = left - 1; int rightPtr = right + 1; while (true) { while (leftPtr &lt; right &amp;&amp; array[++leftPtr] &lt; pivot); while (rightPtr &gt; left &amp;&amp; array[--rightPtr] &gt; pivot); if (leftPtr &gt;= rightPtr) break; else swap(leftPtr, rightPtr); } return leftPtr; } </code></pre> <p>And I've written this method based on <a href="https://en.wikipedia.org/wiki/Selection_algorithm#Partition-based_general_selection_algorithm" rel="nofollow">Wikipedia's pseudocode</a>:</p> <pre><code>public int kthSmallest(int left, int right, int k){ if(left &gt;= right){ return array[left]; } int pivotNewIndex = partition(left, right, array[right]); int pivotDist = pivotNewIndex - left - 1; if(pivotDist == k){ return array[pivotNewIndex]; } else if(k &lt; pivotNewIndex){ return kthSmallest(k, left, pivotNewIndex - 1); } else{ return kthSmallest(k - pivotDist, pivotNewIndex + 1, right); } } </code></pre> <p>But when I call <code>kthSmallest()</code> with randomly generated arrays of integers, about half the time it returns the wrong value. For example:</p> <pre><code>array: [45, 60, 24, 82, 87, 79, 16, 32, 59, 83, 20, 2, 1, 50, 11, 79, 72, 32, 0, 48, 69, 74, 22, 6, 96] expected result when k=4: 11 actual result when k=4: 87 </code></pre> <p>What am I doing wrong?</p>
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      1. VTUpMod
    1. COAre we assuming that you can't sort with quicksort, then just jump to the index? If not, output the array to make sure it's actually in the correct order.
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      1. USThomas
    2. CONo, sorting is off-limits.
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