Question

I’m writing a sudoku backtracking solver, it’s getting stuck and I don’t understand why. I think my recursive calls are alright. What I’m missing?

Input is read from input.txt file with the grid initial layout in a single line:

input.txt:

004020000201950070090004852005490001006000900800051300958100020010072608000080500

Edit: I mean ‘stuck’ as not finishing its solving of the grid

This is a sample output:

current move count is 6
3 6 4 7 2 8 1 9 0 
2 0 1 9 5 0 0 7 0 
0 9 0 0 0 4 8 5 2 
0 0 5 4 9 0 0 0 1 
0 0 6 0 0 0 9 0 0 
8 0 0 0 5 1 3 0 0 
9 5 8 1 0 0 0 2 0 
0 1 0 0 7 2 6 0 8 
0 0 0 0 8 0 5 0 0 

Program:

package sudoku;

import java.io.BufferedReader;
import java.io.File;
import java.io.FileReader;
import java.io.IOException;




public class Main {

 static boolean checkRow( int row, int num, int grid[][])
   {
      for( int col = 0; col < 9; col++ )
         if( grid[row][col] == num )
            return false ;

      return true ;
   }

static boolean checkCol( int col, int num, int grid[][] )
   {
      for( int row = 0; row < 9; row++ )
         if( grid [row][col] == num )
            return false ;

      return true ;
   }


static boolean checkBox( int row, int col, int num, int grid[][] )
   {
      row = (row / 3) * 3 ;
      col = (col / 3) * 3 ;

      for( int r = 0; r < 3; r++ ){
         for( int c = 0; c < 3; c++ ){
            if( grid[row+r][col+c] == num )
            return false ;          
         }
      }
      return true ;
   }


  static void printSolvedGrid(int grid[][]){

       for (int i=0; i<grid.length; i++){
            for (int j=0; j<grid.length;j++){
              System.out.print(grid[i][j]+" ");
            } System.out.println();
        }

  }


   static int moveCounter=0;

   static boolean solve(int row, int col, int [][]grid){

       if (row>=grid.length){

           System.out.println("solution found");
           printSolvedGrid(grid);

       }

       if( grid[row][col] != 0 ){
            next( row, col, grid ) ;
       }

       else {
         // Find a valid number for the empty cell
         for( int num = 1; num < 10; num++ )
         {
            if( checkRow(row,num,grid) && checkCol(col,num,grid) && checkBox(row,col,num,grid) )
            {
               grid[row][col] = num ;
               moveCounter++;
               System.out.println("current move count is " + moveCounter);
               printSolvedGrid(grid);
               next( row, col, grid );
               return true;
            }
         }

       }
       return false;
    }


  public static void next( int row, int col, int [][] grid )
   {
      if( col < 8 ) //pass to next col
         solve( row, col + 1, grid ) ;
      else //pass to next row 
         solve( row + 1, 0, grid ) ;
   }







    public static void main(String[] args) throws IOException {

        BufferedReader br = new BufferedReader(new FileReader(new File("input.txt")));
        char gridChar[] = br.readLine().toCharArray();

       int [][] grid = new int [9][9];

        int gridCharIndex=0;
        for (int i=0; i<grid.length; i++){
            for (int j=0; j<grid.length;j++){

              grid[i][j]= Integer.parseInt(gridChar[gridCharIndex++]+"");
              System.out.print(grid[i][j]+" ");
            } System.out.println();
        }

        solve(0,0, grid);
        }//end method main
    }//end class Main

Answer 1

Fixed:

I took away the boolean and return statements 🙂

BTW, it appears my original test case doesn’t have solution, but these two inputs do yield one:

input1.txt

900576813630090002005000900001004730200000008076900500003000600400050091712869005 

input2.txt (this one is harder)

200609050604083000000700000048000200000010000001000490000001000000970103020408005 

code:

static void solve(int row, int col, int [][]grid){

       if (row>=grid.length){

           System.out.println("solution found");
           printSolvedGrid(grid);
           System.exit(0);

       }

       if( grid[row][col] != 0 ){
            next( row, col, grid ) ;
       }

       else {
         // Find a valid number for the empty cell
         for( int num = 1; num < 10; num++ )
         {
            if( checkRow(row,num,grid) && checkCol(col,num,grid) && checkBox(row,col,num,grid) )
            {
               grid[row][col] = num ;
               moveCounter++;
               System.out.println("current move count is " + moveCounter);
               printSolvedGrid(grid);
               next( row, col, grid );

            }
         }

         grid[row][col] = 0 ;
       }

    }