A solver for Sudoku

SudokuSolver.java

public class SudokuSolver {
  int[] board = new int[81];
  char[] in = null;

  boolean[][] findLimits() {
    boolean[][] ok = new boolean[81][9];
    for (int i = 0; i < 81; i++) {
      for (int j = 0; j < 9; j++) {
        ok[i][j] = true;
      }
    }

    for (int i = 0; i < 81; i++) {
      limitOthersFrom(i, ok);
    }
    return ok;
  }

  int nextMove(boolean[][] ok) {
    int minval = 100, minidx = 0;
    for (int idx = 0; idx < 81; idx++) {
      int numChoices = countChoices(idx, ok);
      if (numChoices > 0 && numChoices < minval) {
        minval = numChoices;
        minidx = idx;
      }
    }
    return minidx;
  }

  void limitOthersFrom(int idx, boolean[][] ok) {
    int r = idx / 9;
    int c = idx % 9;

    if (board[idx] == -1) {
      return;
    }

    int br = r/3;
    int bc = c/3;
    int val = board[idx];

    // limit itself
    for (int i = 0; i < 9; i++) {
      ok[idx][i] = false;
    }

    // limit row
    for (int cc = 0; cc < 9; cc++) {
      ok[r * 9 + cc][val] = false;
    }

    // limit col
    for (int rr = 0; rr < 9; rr++) {
      ok[rr * 9 + c][val] = false;
    }

    // limit block
    for (int i = 0; i < 9; i++) {
      int rr = br*3 + i/3;
      int cc = bc*3 + i%3;
      ok[rr * 9 + cc][val] = false;
    }
  }

  public void display() {
    for (int i = 0; i < 9; i++) {
      for (int j = 0; j < 9; j++) {
        System.out.print((board[i * 9 + j] + 1) + " ");
      }
      System.out.println();
    }
    System.out.println();
  }

  public int countChoices(int idx, boolean[][] ok) {
    int count = 0;
    for (int i = 0; i < 9; i++) {
      if (ok[idx][i]) {
        count++;
      }
    }
    return count;
  }

  public boolean solve() {
    boolean[][] ok = findLimits();
    int idx = nextMove(ok);

    if (countChoices(idx, ok) == 0) {
      display();
      return true;
    }

    for (int i = 0; i < 9; i++) {
      if (ok[idx][i]) {
        board[idx] = i;
        boolean res = solve();
        if (res) {
          return true;
        }
        board[idx] = -1;
      }
    }

    return false;
  }

  public void read(String in) {
    if (in == null || in.length() != 81) {
      throw new IllegalArgumentException();
    }
    this.in = in.toCharArray();

    for (int idx = 0; idx < in.length(); idx++) {
      char ch = in.charAt(idx);
      if (ch >= '1' && ch <= '9') {
        board[idx] = ch - '1';
      } else {
        board[idx] = -1;
      }
    }
  }
  
  public static void main(String[] args) {
    SudokuSolver ss = new SudokuSolver();
    String in = "......52..8.4......3...9...5.1...6..2..7........3.....6...1..........7.4.......3.";
    ss.read(in);
    long from = System.currentTimeMillis();
    ss.solve();
    long to = System.currentTimeMillis();
    float delta = (float)(to - from)/1000;
    System.out.println(delta);
  }
}