Generate all valid parenthesis of length k

void fun(int n,string cur,int open,int close,vector <string> &ans){
    if(open>n || close>n) return;
    if(open==n && close==n){ ans.push_back(cur); return;}
    if(open==close){
        fun(n,cur+"(",open+1,close,ans);
    }
    else if(open>close){
        fun(n,cur+"(",open+1,close,ans);
        fun(n,cur+")",open,close+1,ans);
    }
}
class Solution
{
    public:
    vector<string> AllParenthesis(int n)
    {
        vector <string> ans;
        fun(n,"",0,0,ans);
        return ans;
    }
};