leetcode 22. Generate Parentheses

void generateParent(vector<string>& parenthesis , string cur ,int left,int n){    if(cur.length() == 2*n){        parenthesis.push_back(cur);        return;    }    if(left> (cur.length()-left) ){        generateParent(parenthesis,cur+')',left,n);    }    if(left<n){        generateParent(parenthesis,cur+'(',left+1,n);    }}vector<string> generateParenthesis(int n) {    vector<string> ret;    generateParent(ret, "",  0,n);    return ret;}