Generate all valid parenthesis of length k

def fun(open,close,cur,ans):
    if open==0 and close==0:
        ans.add(cur)
    if open>0:
        fun(open-1,close,cur+'(',ans)
    if close>open:
        fun(open,close-1,cur+')',ans)

class Solution:
    def generateParenthesis(self, n: int) -> List[str]:
        ans=set()
        fun(n,n,'',ans)
        return list(ans)