leetcode - Generate Parentheses

class Solution {
public:
void dfs(int n, int before, int left, string cur, vector<string> &rlt){
int right = before - left;
if (left<n){
dfs(n, before+1, left+1, cur+"(", rlt);
}
if (right<left){
if (before == 2*n-1)
rlt.push_back(cur+")");
else dfs(n,before+1, left, cur+")", rlt);
}
}
vector<string> generateParenthesis(int n) {
// Start typing your C/C++ solution below
// DO NOT write int main() function
vector<string> rlt;
dfs(n, 0, 0, "", rlt);
return rlt;
}
};