SudokuSolver.java

package sudoku;

/**
 * Created by Raymond on 2015-08-01.
 */
public class Solver {

    private static final int MAX_ROW = 9;
    private static final int MAX_COL = 9;

    public boolean solve(int[][] board, int row, int col) {
        // Return condition, the board has reached the end
        if (row == MAX_ROW) return true;

        // Only modify non-filled cell
        if (board[row][col] != 0) {
            if (col == 8) {
                if (solve(board, row + 1, 0)) return true;
            } else {
                if (solve(board, row, col + 1)) return true;
            }
        }

        //Try number 1-9
        for (int num = 1; num <= 9; num++) {
            if (isValid(num, board, row, col)) {
                board[row][col] = num;
                if (col == 8) {
                    if (solve(board, row + 1, 0)) return true;
                } else {
                    if (solve(board, row, col + 1)) return true;
                }
            }
        }
        board[row][col] = 0; //Backtrack here, reset the cell to 0
        return false; 
    }

    /**
     * 
     * @param num
     * @param board
     * @param row
     * @param col
     * @return
     */
    protected boolean isValid(int num, int[][] board, int row, int col) {
        return isRowValid(num, board, row) && isColValid(num, board, col) && isBoxValid(num, board, row, col);
    }

    /**
     * 
     * @param num
     * @param board
     * @param row
     * @return
     */
    private boolean isRowValid(int num, int[][] board, int row) {
        for (int col = 0; col < MAX_COL; col++) {
            if (num == board[row][col]) {
                return false;
            }
        }
        return true;
    }

    /**
     * 
     * @param num
     * @param board
     * @param col
     * @return
     */
    private boolean isColValid(int num, int[][] board, int col) {
        for (int row = 0; row < MAX_ROW; row++) {
            if (num == board[row][col]) {
                return false;
            }
        }
        return true;
    }

    /**
     * 
     * @param num
     * @param board
     * @param row
     * @param col
     * @return
     */
    private boolean isBoxValid(int num, int[][] board, int row, int col) {
        int startingRow = row/3;
        int startingCol = col/3;

        for (int i = startingRow * 3; i < startingRow + 3; i++) {
            for (int j = startingCol * 3; j < startingCol + 3; j++) {
                if (num == board[i][j]) {
                    return false;
                }
            }
        }
        return true;
    }

}