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Sudoku solver in java, using backtracking and recursion

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.

The general algorithm of the program is the following:

1. Find the number of zeros in the sudoku.

2. 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).

3. Then I make a recursive call, end when there are no more zeroes (n is the number of zeros).

4. If the program reaches a point that it gets stuck, I must backtrack to change a piece. But how is this possible?

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.

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.

public static int[][] getSolution(int[][] grid) {
    for (int i = 0; i < 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 <= 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

Answer 1

A nudge in the right direction. Your approach needs a little tweaking since you’re not keeping track of all the information you need to solve the grid.

private static int[][] getSolution(int n) {
    if (n == 0) {
       return grid;        
    }//end if

    Cell cell = getNextEmpty();
    boolean fits = false;
    for (int i = 0; i <= 9; i++) {
        fits = insertNumber(cell, i);
        if (fits) {
            break; // means i fits in Cell
        } else {
            // i doesn't fit... try the next i
            // don't need to do anything
        }
    }//end for
    if (!fits) {
        // There are no numbers that fit in this Cell
        // What should happen?
        // Did I make a bad guess?
        // How do I BACKTRACK and correct a previous guess?
    }
    return getSolution(n - 1);
}//end getSolution