Returns a list of roots of the graph.

        Examples
        --------
        >>> from pgmpy.base import DAG
        >>> graph = DAG([('A', 'B'), ('B', 'C'), ('B', 'D'), ('E', 'B')])
        >>> graph.get_roots()
        ['A', 'E']

def get_roots(self):
        """
        Returns a list of roots of the graph.

        Examples
        --------
        >>> from pgmpy.base import DAG
        >>> graph = DAG([('A', 'B'), ('B', 'C'), ('B', 'D'), ('E', 'B')])
        >>> graph.get_roots()
        ['A', 'E']
        """
        return [node for node, in_degree in dict(self.in_degree()).items() if in_degree == 0]