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  1. PO
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    <p>I think you can use a 1D array, in much the same way a 1D array can model a binary tree. For example, to look at the value below a number, add 9 to the index.</p> <p>I just made this up, but could something like this work? </p> <pre><code>private boolean makePuzzle(int [] puzzle, int i) { for (int x = 0; x&lt; 10 ; x++) { if (//x satisfies all three conditions for the current square i) { puzzle[i]=x; if (i==80) return true //terminal condition, x fits in the last square else if makePuzzle(puzzle, i++);//find the next x return true; }// even though x fit in this square, an x couldn't be // found for some future square, try again with a new x } return false; //no value for x fit in the current square } public static void main(String[] args ) { int[] puzzle = new int[80]; makePuzzle(puzzle,0); // print out puzzle here } </code></pre> <p>Edit: its been a while since I've used arrays in Java, sorry if I screwed up any syntax. Please consider it pseudo code :)</p> <p>Here is the code as described below in my comment.</p> <pre><code>public class Sudoku { public int[] puzzle = new int[81]; private void makePuzzle(int[] puzzle, int i) { for (int x = 1; x&lt; 10 ; x++) { puzzle[i]=x; if(checkConstraints(puzzle)) { if (i==80)//terminal condition { System.out.println(this);//print out the completed puzzle puzzle[i]=0; return; } else makePuzzle(puzzle,i+1);//find a number for the next square } puzzle[i]=0;//this try didn't work, delete the evidence } } private boolean checkConstraints(int[] puzzle) { int test; //test that rows have unique values for (int column=0; column&lt;9; column++) { for (int row=0; row&lt;9; row++) { test=puzzle[row+column*9]; for (int j=0;j&lt;9;j++) { if(test!=0&amp;&amp; row!=j&amp;&amp;test==puzzle[j+column*9]) return false; } } } //test that columns have unique values for (int column=0; column&lt;9; column++) { for(int row=0; row&lt;9; row++) { test=puzzle[column+row*9]; for (int j=0;j&lt;9;j++) { if(test!=0&amp;&amp;row!=j&amp;&amp;test==puzzle[column+j*9]) return false; } } } //implement region test here int[][] regions = new int[9][9]; int[] regionIndex ={0,3,6,27,30,33,54,57,60}; for (int region=0; region&lt;9;region++) //for each region { int j =0; for (int k=regionIndex[region];k&lt;regionIndex[region]+27; k=(k%3==2?k+7:k+1)) { regions[region][j]=puzzle[k]; j++; } } for (int i=0;i&lt;9;i++)//region counter { for (int j=0;j&lt;9;j++) { for (int k=0;k&lt;9;k++) { if (regions[i][j]!=0&amp;&amp;j!=k&amp;&amp;regions[i][j]==regions[i][k]) return false; } } } return true; } public String toString() { String string= ""; for (int i=0; i &lt;9;i++) { for (int j = 0; j&lt;9;j++) { string = string+puzzle[i*9+j]; } string =string +"\n"; } return string; } public static void main(String[] args) { Sudoku sudoku=new Sudoku(); sudoku.makePuzzle(sudoku.puzzle, 0); } } </code></pre>
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    1. POJava Sudoku Generator(easiest solution)
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    1. COJust thought of this... When the array is initialized it will be full of zeros, which kind of screws things up. Maybe it would be better to use 1...10 for the possible sudoku values, instead of 0...9. When you are done generating you can just decrement each value.
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    2. COJust thought of something else... I obviously don't know anything about sudoku :) Apparently 0 isn't a valid value in a sudoku puzzle! Who knew? I cleaned up the above code though, and it seemed to work after I fixed all the obvious mistakes :) You just need to write the constraints properly.
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    3. COOk, actually got this working, so here's some code. It works, but it only tests columns and rows. You'll have to figure out the region test yourself :) If you decide to convert it to a 2d array for this test check out the toString() method. Once you add that, this will generate all valid sudoku grids. I'll edit the original answer to include the new code.
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