_37.java

package com.fishercoder.solutions;

/**
 * Write a program to solve a Sudoku puzzle by filling the empty cells.
 * Empty cells are indicated by the character '.'.
 * You may assume that there will be only one unique solution.
  * A sudoku puzzle...
 * ...and its solution numbers marked in red.
 */
public class _37 {

    public void solveSudoku(char[][] board) {
        if (board == null || board.length == 0) return;
        solve(board);
    }

    private boolean solve(char[][] board) {
        for (int i = 0; i < board.length; i++) {
            for (int j = 0; j < board[0].length; j++) {
                if (board[i][j] == '.') {
                    for (char c = '1'; c <= '9'; c++) {//try 1 to 9
                        if (isValid(board, i, j, c)) {
                            board[i][j] = c;

                            if (solve(board)) return true;
                            else board[i][j] = '.';//recover it to be '.'
                        }
                    }
                    return false;
                }
            }
        }
        return true;
    }

    private boolean isValid(char[][] board, int row, int col, char c) {
        for (int i = 0; i < 9; i++) {
            if (board[i][col] != '.' && board[i][col] == c) return false;//check row
            if (board[row][i] != '.' && board[row][i] == c) return false;//check column
            if (board[3 * (row / 3) + i / 3][3 * (col / 3) + i % 3] != '.' && board[3 * (row / 3) + i / 3][3 * (col / 3) + i % 3] == c)
                return false; //check 3*3 block
        }
        return true;
    }

}