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POTips implementing permutation algorithm in Java
 

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    <p>As part of a school project, I need to write a function that will take an integer N and return a two-dimensional array of every permutation of the array {0, 1, ..., N-1}. The declaration would look like public static int[][] permutations(int N).</p> <p>The algorithm described at <a href="http://www.usna.edu/Users/math/wdj/book/node156.html" rel="nofollow">http://www.usna.edu/Users/math/wdj/book/node156.html</a> is how I've decided to implement this.</p> <p>I wrestled for quite a while with arrays and arrays of ArrayLists and ArrayLists of ArrayLists, but so far I've been frustrated, especially trying to convert a 2d ArrayList to a 2d array.</p> <p>So I wrote it in javascript. This works:</p> <pre><code>function allPermutations(N) { // base case if (N == 2) return [[0,1], [1,0]]; else { // start with all permutations of previous degree var permutations = allPermutations(N-1); // copy each permutation N times for (var i = permutations.length*N-1; i &gt;= 0; i--) { if (i % N == 0) continue; permutations.splice(Math.floor(i/N), 0, permutations[Math.floor(i/N)].slice(0)); } // "weave" next number in for (var i = 0, j = N-1, d = -1; i &lt; permutations.length; i++) { // insert number N-1 at index j permutations[i].splice(j, 0, N-1); // index j is N-1, N-2, N-3, ... , 1, 0; then 0, 1, 2, ... N-1; then N-1, N-2, etc. j += d; // at beginning or end of the row, switch weave direction if (j &lt; 0 || j &gt;= N) { d *= -1; j += d; } } return permutations; } } </code></pre> <p>So what's the best strategy to port that to Java? Can I do it with just primitive arrays? Do I need an array of ArrayLists? Or an ArrayList of ArrayLists? Or is there some other data type that's better? Whatever I use, I need to be able to convert it back into a an array of primitive arrays.</p> <p>Maybe's there a better algorithm that would simplify this for me...</p> <p>Thank you in advance for your advice!</p>
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    1. CO`allPermutations(1)` will recurse until a `StackOverflow` occurs.
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    2. COI am also living that life. Best of Luck bro.
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