Generate Parentheses

class Solution:
    def generateParenthesis(self, n: int) -> List[str]:
        # Invalid:
        # more closing than opening
        # num opening parens greater than n
        valid_parens = []

        def find_valid_parens(curr_str, num_left, num_right):
            # Check invalid
            # If invalid, return to prune subtree
            if num_right > num_left or num_left > n:
                return

            # Check valid
            # If leaf node, add to valid_parens and return
            if num_left == n and num_right == n:
                valid_parens.append(curr_str)
                return

            # Recurse To keep going down tree
            # Add left paren
            find_valid_parens(curr_str + "(", num_left + 1, num_right)
            # Add right paren
            find_valid_parens(curr_str + ")", num_left, num_right + 1)

        find_valid_parens('', 0, 0)
        return valid_parens