hasUniqueSolution static method

bool hasUniqueSolution(List<List<int>> sudoku)

Returns true if the sudoku has only 1 solution.

InvalidSudokuConfigurationException is thrown if the configuration of the sudoku is not valid.

Implementation

static bool hasUniqueSolution(List<List<int>> sudoku) {
  bool legal(List<List<int>> puzzle, int x, int y, int num) {
    var i = 9;
    var size = 9;
    var rowStart = x ~/ 3 * 3;
    var colStart = y ~/ 3 * 3;
    for (i = 0; i < size; i++) {
      if (puzzle[x][i] == num) {
        return false;
      }
      if (puzzle[i][y] == num) {
        return false;
      }
      if (puzzle[rowStart + (i % 3)][colStart + (i ~/ 3)] == num) {
        return false;
      }
    }
    return true;
  }

  int checkUniqueSolution(List<List<int>> puzzle, int x, int y, int count) {
    var num = 9;
    var size = 9;
    if (x == size) {
      x = 0;
      if (++y == size) {
        return 1 + count;
      }
    }
    if (puzzle[x][y] != 0) {
      return checkUniqueSolution(puzzle, x + 1, y, count);
    }
    for (num = 1; (num <= size) && (count < 2); num++) {
      if (legal(puzzle, x, y, num)) {
        puzzle[x][y] = num;
        count = checkUniqueSolution(puzzle, x + 1, y, count);
      }
    }
    puzzle[x][y] = 0;
    return count;
  }

  if (!isValidConfiguration(sudoku)) {
    throw InvalidSudokuConfigurationException();
  }
  return checkUniqueSolution(sudoku, 0, 0, 0) == 1;
}