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    <p><strong>Some answers to this question are good but rather inefficient (O(N^2)).</strong> </p> <p>This is so, because the path is constructed from scratch for every <code>Positionen</code> element. The average path length is N/2 and there are N <code>Positionen</code> elements. That means that N*N/2 operations are needed as minimum to construct all paths -- and this is quadratical time complexity.</p> <p><strong>Here is a more efficient O(N*log(N)) -- could be even (O(N) -- linear) solution in case it is acceptable for the <code>Positionen</code> elements in the output to be unsorted</strong>:</p> <pre><code>&lt;xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform" xmlns:ext="http://exslt.org/common" exclude-result-prefixes="ext"&gt; &lt;xsl:output omit-xml-declaration="yes" indent="yes"/&gt; &lt;xsl:strip-space elements="*"/&gt; &lt;xsl:key name="kChildren" match="Positionen" use="Parent"/&gt; &lt;xsl:template match="node()|@*"&gt; &lt;xsl:copy&gt; &lt;xsl:apply-templates select="node()|@*"/&gt; &lt;/xsl:copy&gt; &lt;/xsl:template&gt; &lt;xsl:template match="/"&gt; &lt;xsl:variable name="vrtfPass1"&gt; &lt;xsl:apply-templates select="node()|@*"/&gt; &lt;/xsl:variable&gt; &lt;xsl:variable name="vPass1" select="ext:node-set($vrtfPass1)"/&gt; &lt;xsl:apply-templates select="$vPass1/*" mode="pass2"/&gt; &lt;/xsl:template&gt; &lt;xsl:template match="/*"&gt; &lt;xsl:copy&gt; &lt;xsl:apply-templates select="Positionen[not(number(Parent))]" mode="path"/&gt; &lt;/xsl:copy&gt; &lt;/xsl:template&gt; &lt;xsl:template match="Positionen" mode="path"&gt; &lt;xsl:param name="pPath"/&gt; &lt;xsl:copy&gt; &lt;xsl:apply-templates select="node()|@*"/&gt; &lt;Path&gt;&lt;xsl:value-of select="concat($pPath, ID)"/&gt;&lt;/Path&gt; &lt;/xsl:copy&gt; &lt;xsl:apply-templates select="key('kChildren', ID)" mode="path"&gt; &lt;xsl:with-param name="pPath" select="concat($pPath, ID, '/')"/&gt; &lt;/xsl:apply-templates&gt; &lt;/xsl:template&gt; &lt;xsl:template match="/*" mode="pass2"&gt; &lt;xsl:copy&gt; &lt;xsl:apply-templates select="node()|@*"&gt; &lt;xsl:sort select="ID" data-type="number"/&gt; &lt;/xsl:apply-templates&gt; &lt;/xsl:copy&gt; &lt;/xsl:template&gt; &lt;/xsl:stylesheet&gt; </code></pre> <p><strong>when this transformation is applied on the provided XML document</strong>:</p> <pre><code>&lt;Positions&gt; &lt;Positionen&gt; &lt;ID&gt;1&lt;/ID&gt; &lt;Parent&gt;&lt;/Parent&gt; &lt;/Positionen&gt; &lt;Positionen&gt; &lt;ID&gt;2&lt;/ID&gt; &lt;Parent&gt;1&lt;/Parent&gt; &lt;/Positionen&gt; &lt;Positionen&gt; &lt;ID&gt;3&lt;/ID&gt; &lt;Parent&gt;1&lt;/Parent&gt; &lt;/Positionen&gt; &lt;Positionen&gt; &lt;ID&gt;4&lt;/ID&gt; &lt;Parent&gt;2&lt;/Parent&gt; &lt;/Positionen&gt; &lt;Positionen&gt; &lt;ID&gt;5&lt;/ID&gt; &lt;Parent&gt;4&lt;/Parent&gt; &lt;/Positionen&gt; &lt;Positionen&gt; &lt;ID&gt;6&lt;/ID&gt; &lt;Parent&gt;2&lt;/Parent&gt; &lt;/Positionen&gt; &lt;/Positions&gt; </code></pre> <p><strong>the wanted, correct result is produced:</strong></p> <pre><code>&lt;Positions&gt; &lt;Positionen&gt; &lt;ID&gt;1&lt;/ID&gt; &lt;Parent/&gt; &lt;Path&gt;1&lt;/Path&gt; &lt;/Positionen&gt; &lt;Positionen&gt; &lt;ID&gt;2&lt;/ID&gt; &lt;Parent&gt;1&lt;/Parent&gt; &lt;Path&gt;1/2&lt;/Path&gt; &lt;/Positionen&gt; &lt;Positionen&gt; &lt;ID&gt;3&lt;/ID&gt; &lt;Parent&gt;1&lt;/Parent&gt; &lt;Path&gt;1/3&lt;/Path&gt; &lt;/Positionen&gt; &lt;Positionen&gt; &lt;ID&gt;4&lt;/ID&gt; &lt;Parent&gt;2&lt;/Parent&gt; &lt;Path&gt;1/2/4&lt;/Path&gt; &lt;/Positionen&gt; &lt;Positionen&gt; &lt;ID&gt;5&lt;/ID&gt; &lt;Parent&gt;4&lt;/Parent&gt; &lt;Path&gt;1/2/4/5&lt;/Path&gt; &lt;/Positionen&gt; &lt;Positionen&gt; &lt;ID&gt;6&lt;/ID&gt; &lt;Parent&gt;2&lt;/Parent&gt; &lt;Path&gt;1/2/6&lt;/Path&gt; &lt;/Positionen&gt; &lt;/Positions&gt; </code></pre> <p><strong>Do Note</strong>:</p> <p>Every Path is produced by adding the current <code>ID</code> to the path of the parent (which is calculated only once) -- an O(1) operation. Fot the total of N paths this is O(N).</p> <p>The final sorting makes the time complexity O(N*log(N)) -- still much better than quadratical.</p>
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