Create a string to pretty-print this trie to standard output.

def pretty_print_str(self):
        '''
        Create a string to pretty-print this trie to standard output.
        '''
        retval = ''
        # dfs
        todo = [self.root]
        while todo:
            current = todo.pop()
            for char in reversed(sorted(current.keys())):
                todo.append(current[char])
            indent = ' ' * (current.depth * 2)
            retval += indent + current.__unicode__() + '\n'
        return retval.rstrip('\n')