Sudoku Solver.java

public class Solution {
    static final int N = 9;

    public void solveSudoku(char[][] board) {
        solveSudoku(board, 0);
    }

    private boolean solveSudoku(char[][] board, int n) {
        if (n == N * N) 
            return true;
        int x = n / N;
        int y = n % N;
        if (board[x][y] != '.') 
            return solveSudoku(board, n + 1);
        int xi, yi;
        int[] choices = new int[N];
        for (int i = 0; i < N; i++) {
            if (board[x][i] != '.') 
                choices[board[x][i] - '1'] = 1;
            if (board[i][y] != '.') 
                choices[board[i][y] - '1'] = 1;
            xi = x / 3 * 3 + i / 3;
            yi = y / 3 * 3 + i % 3;
            if (board[xi][yi] != '.') 
                choices[board[xi][yi] - '1'] = 1;
        }
        for (int i = 0; i < N; i++) 
            if (choices[i] == 0) {
                board[x][y] = (char) ('1' + i);
                if (solveSudoku(board, n + 1)) 
                    return true;
                board[x][y] = '.';
            }
        return false;
    }

}