Sudoku Solver

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 Solution {
    public void solveSudoku(char[][] board) {
        solve(board);
    }
 
    private boolean solve(char[][] board) {
        for (int i=0; i<9; ++i) {
            for (int j=0; j<9; ++j) {
                if ('.' == board[i][j]) {
                    for (int k=1; k<=9; ++k) {
                        board[i][j] = (char)('0'+k);
                        if (validate(board, i, j) && solve(board))
                            return true;
                    }
                    board[i][j] = '.';
                    return false;
                }
            }
        }
        return true;
    }
     
    private boolean validate(char[][] board, int x, int y) {
        for (int i=0; i<9; i++)
            if (i!=x && board[i][y]==board[x][y])
                return false;
         
        for (int j = 0; j<9; j++)
            if (j!=y && board[x][j]==board[x][y])
                return false;
         
        for (int i=3*(x/3); i<3*(x/3)+3; i++)
            for (int j=3*(y/3); j<3*(y/3)+3; j++)
                if (i!=x && j!=y && board[i][j]==board[x][y])
                    return false;
         
        return true;
    }
}