private static void solve(Grid grid, List<Grid> solutions) { // Return if there is already more than two solution if (solutions.size() >= 2) { return; } // Find first empty cell int loc = grid.findEmptyCell(); // If no empty cells are found,a solution is found if (loc < 0) { solutions.add(grid.clone()); return; } // Try each of the 9 digits in this empty cell for (int n = 1; n < 10; n++) { if (grid.set(loc, n)) { // With this cell set,work on the next cell solve(grid, solutions); // Clear the cell so that it can be filled with another digit grid.clear(loc); } } }