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StackOverflow2013
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    <p>Here is one in C#, which should work even with repeated characters. For example on "banana" for permutations of length 2 it gives:</p> <blockquote> <p>ba bn ab aa an nb na nn</p> </blockquote> <p>The basic idea is to fix the first character, then form all permutations of length k-1, then prepend the character to those k-1 length permutations. To deal with duplicate characters, we keep track of the count left (i.e the ones which can be used for sub-permutations).</p> <p>Not exemplary code, but should give you the idea. (If you find bugs, let me know and I can edit).</p> <pre><code>static List&lt;string&gt; Permutations(Dictionary&lt;char, int&gt; input, int length) { List&lt;string&gt; permutations = new List&lt;string&gt;(); List&lt;char&gt; chars = new List&lt;char&gt;(input.Keys); // Base case. if (length == 0) { permutations.Add(string.Empty); return permutations; } foreach (char c in chars) { // There are instances of this character left to use. if (input[c] &gt; 0) { // Use one instance up. input[c]--; // Find sub-permutations of length length -1. List&lt;string&gt; subpermutations = Permutations(input, length - 1); // Give back the instance. input[c]++; foreach (string s in subpermutations) { // Prepend the character to be the first character. permutations.Add(s.Insert(0,new string(c,1))); } } } return permutations; } </code></pre> <p>And here is the full program I have, to use it:</p> <pre><code>using System; using System.Collections.Generic; namespace StackOverflow { class Program { static void Main(string[] args) { List&lt;string&gt; p = Permutations("abracadabra", 3); foreach (string s in p) { Console.WriteLine(s); } } static List&lt;string&gt; Permutations(string s, int length) { Dictionary&lt;char, int&gt; input = new Dictionary&lt;char, int&gt;(); foreach (char c in s) { if (input.ContainsKey(c)) { input[c]++; } else { input[c] = 1; } } return Permutations(input, length); } static List&lt;string&gt; Permutations(Dictionary&lt;char, int&gt; input, int length) { List&lt;string&gt; permutations = new List&lt;string&gt;(); List&lt;char&gt; chars = new List&lt;char&gt;(input.Keys); if (length == 0) { permutations.Add(string.Empty); return permutations; } foreach (char c in chars) { if (input[c] &gt; 0) { input[c]--; List&lt;string&gt; subpermutations = Permutations(input, length - 1); input[c]++; foreach (string s in subpermutations) { permutations.Add(s.Insert(0,new string(c,1))); } } } return permutations; } } } </code></pre>
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    1. COThanks for this code.
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    2. COI studied it and it's pretty cool.
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    3. CO@Y_Y: Glad it helped! It was fun to revisit this one after a long time.
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