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    <p>Avoid using <code>ArrayList</code>, prefer generic collection such as <code>List&lt;Int64&gt;</code> because you conversion may be quite expensive.</p> <p>There is a part in your program which is in <em>O(n^3)</em>. It can be reduce to <em>O(n^2)</em> easily. Just replace the following code (<em>O(n)</em> because of <code>List&lt;T&gt;.Contains</code>):</p> <pre><code>if(listOfSums.Contains(num) == false) { listOfSums.Add(num); } </code></pre> <p>by:</p> <pre><code>listOfSums.Add(num); </code></pre> <p>where <code>listOfSums</code> is now a <code>HashSet&lt;Int64&gt;</code> (it's used to have a collection without duplicates).</p> <p>Moreover, you can reduce the time for populating the <code>listOfSums</code> by a factor of 2 by just noticing that if you exchange <code>i</code> and <code>a</code>, the <code>sum</code> is the same, then, only one of the two combinations is added to the <code>listOfSums</code>.</p> <p>You can replace:</p> <pre><code>for (int i = 1; i &lt; listOfAbundantNumbers.Count; i++) for (int a = 0; a &lt; listOfAbundantNumbers.Count; a++) </code></pre> <p>by;</p> <pre><code>for (int i = 1; i &lt; listOfAbundantNumbers.Count; i++) for (int a = 0; a &lt;= i; a++) </code></pre> <p>The final code is:</p> <pre><code>private static void Main() { List&lt;Int64&gt; listOfAbundantNumbers = new List&lt;Int64&gt;(); HashSet&lt;Int64&gt; listOfSums = new HashSet&lt;Int64&gt;(); long total = 0; for (int i = 1; i &lt; 20162; i++) { if (isAbundandt(i)) { listOfAbundantNumbers.Add(i); } total += i; } for (int i = 0; i &lt; listOfAbundantNumbers.Count; i++) for (int a = 0; a &lt;= i; a++) { long temp1 = Convert.ToInt64(listOfAbundantNumbers[i]); long temp2 = Convert.ToInt64(listOfAbundantNumbers[a]); long num = temp1 + temp2; listOfSums.Add(num); } for (int i = 1; i &lt; listOfAbundantNumbers.Count; i++) { long temp1 = Convert.ToInt64(listOfAbundantNumbers[i]); total -= temp1; } Console.WriteLine(total + ""); } private static List&lt;Int64&gt; divisorList(long input) { List&lt;Int64&gt; divisors = new List&lt;Int64&gt;(); for (long i = 2; i &lt; Math.Round(Math.Sqrt(input), 0, 0); i++) { long temp = input % i; if (temp == 0) { divisors.Add(i); divisors.Add(input / i); } } return divisors; } private static Boolean isAbundandt(long input) { long sum = 0; List&lt;Int64&gt; divisor = divisorList(input); for (int i = 0; i &lt; divisor.Count; i++) { long temp1 = divisor[i]; sum += temp1; } sum++; if (sum &gt; input) { return true; } return false; } </code></pre>
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    1. POProject Euler #23 in C# inefficient code execution time
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    1. POProject Euler #23 in C# inefficient code execution time
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    1. COEuler problems are meant for learning, I would not just give the exact solution for it. A nudge in the right general direction is usually enough.
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    2. CO@JBetter Should I remove my answer?
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    3. COI am not opposed to him leaving the code but others may be. I want to learn more efficient ways to handle what i was doing and wanted to know the downfall of different classes and that's exactly the information your giving me. Besides the code still has a few logical errors in it that havent entirely been pointed out. Someone that was wanting to just copy and paste would be sorely disappointed with the wrong answer.
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