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    <p>In the gist (you accidentally linked both to the same), you have a few typos</p> <pre><code>public static List&lt;Integer&gt; execSort(List&lt;Integer&gt; s) { int start = (s.size()/2)-1; int end = s.size()-1; while( start &gt;= 0){ s = sift(s, start, end); </code></pre> <p><code>sift</code> takes the count as last argument, not the last index, so the argument ought to be <code>s.size()</code> (or <code>end+1</code>) instead of <code>end</code>.</p> <pre><code>public static List&lt;Integer&gt; sift(List&lt;Integer&gt; s, int start, int count){ int root = start; while( ((root*2)+1) &lt; 2 ){ </code></pre> <p>That must be <code>while(root*2+1 &lt; count)</code>, and not <code>&lt; 2</code>.</p> <p>In the code you have here, you have in part the same problem (caused by an odd indexing strategy, I suspect):</p> <pre><code> for(int i = n/2; i&gt;0; i--){ s = downheap(s, i, n); </code></pre> <p>Since you always <code>get(i-1)</code> resp. <code>j-1</code> in <code>downheap</code>, you need an upper bound of <code>s.size()</code> or <code>n+1</code> while building the initial heap.</p> <pre><code> } while(n &gt;= 1){ </code></pre> <p>This loop should only run while <code>n &gt; 1</code>, or you'll swap the smallest element out of place.</p> <pre><code> t= s.remove(0); s.add(0, s.remove(n-1)); s.add(n-1, t); </code></pre> <p>The old root must go in the last place, that's place <code>n</code>, not <code>n-1</code>, <code>s.add(n,t)</code>.</p> <pre><code> n--; s = downheap(s, 1, n); } </code></pre> <p>In <code>downheap</code>, the final</p> <pre><code> s.set(i-1, t); </code></pre> <p>is superfluous, you always swap <code>t</code>, so when that line is reached, the element at <code>i-1</code> already is <code>t</code>.</p>
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    1. USDaniel Fischer
    1. USDaniel Fischer
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    1. COCant believe it was one typo (the while loop) in my original implementation on github. Ironically I did better there than when I was examining other code to re-write it..! This is a brilliant answer, I'm very grateful. Although the last 2 elements are still the wrong way around, I can fix that no doubt! Thanks again.
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      1. USFergus In London
 

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