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POSudoku recursion problematic while backtraking. (Brute Force)
 

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  1. POSudoku recursion problematic while backtraking. (Brute Force)
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    <blockquote> <p><strong>Possible Duplicate:</strong><br> <a href="https://stackoverflow.com/questions/9404673/sudoku-solver-in-java-using-backtracking-and-recursion">Sudoku solver in java, using backtracking and recursion</a> </p> </blockquote> <p>I am creating a program that will solve a sudoku using recursion and brute force. My key problem is that I do not understand how I could concievably make it backtrack one it gets stuck.</p> <p>The general algorithm of the program is the following:</p> <ol> <li><p>Find the number of zeros in the sudoku.</p></li> <li><p>In the location of the first 0 (getNextEmpty method does this), insert a number(insertnumber checks to make sure a value complies with sudoku rules and returns true if it does).</p></li> <li><p>Then I make a recursive call, end when there are no more zeroes (n is the number of zeros).</p></li> <li><p>If the program reaches a point that it gets stuck, I must backtrack to change a piece. But how is this possible? </p></li> </ol> <p>The Cell class actually holds the location of the cell to be adjusted in an array of the format [row, column]. It has methods to return the row, column, or smaller grid associated with that cell.</p> <p>I am not asking for hand-holding or all the code, just a nudge in the right direction will suffice as I am legitimately interested in understanding the recursion.</p> <pre><code>public static int[][] getSolution(int[][] grid) { for (int i = 0; i &lt; 9; i++) { System.arraycopy(grid[i], 0, SolveSudoku.grid[i], 0, 9); }// end for int n = getZeroes(); return getSolution(n); }//end getSolution private static int[][] getSolution(int n) { if (n == 0) { return grid; }//end if Cell cell = getNextEmpty(); boolean fits = false; for (int i = 0; i &lt;= 9; i++) { fits = insertNumber(cell, i); if (fits) { break; }else { //I do not understand what I should do here } }//end for return getSolution(n - 1); }//end getSolution </code></pre>
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    1. CO"brute force" ?
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    2. COyes, the intention is to try every combination until it works. Rather than a calculated approach to solving the sudoku.
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    3. COHave you done some back-of-the-envelope math to find out if this is halfway reasonable to brute strength? I don't know the problem too well, but this strikes me as something that could spin for a few hundred or thousand years if you get a "hard" puzzle. Or much, much longer if you get an impossible one (for which you'll have to enumerate every wrong path).
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