for (int row = 0; row < board.length; row++) {
                for (int col = 0; col < board.length; col++) {
                 // we search an empty cell
                 if (board[row][col] == 0) {
                   // we try possible numbers
                   for (int number = 1; number <= board.length; number++) {
                     if (isLegal(number, number, number)) {
                       // number ok. it respects sudoku constraints
                       board[row][col] = number;

                       if (solve()) { // we start backtracking recursively
                         return true;
                       } else { // if not a solution, we empty the cell and we continue
                         board[row][col] = 0;
                       }
                    }
                   }

                   return false; // we return false
                  }
                }
            }

            return true; // sudoku solved
   	}