DFS Generate Parentheses

思想：

DFS

class Solution {public:    //Generate Parentheses    void generate(vector<string> &res, string s, int n, int l, int r) {        if(l==n) {            s.append(n-r, ')');//add n ')'            res.push_back(s);            return;        }        generate(res, s+'(', n, l+1, r);        if(l>r) generate(res, s+')', n, l, r+1);    }    vector<string> generateParenthesis(int n) {        vector<string> res;        if(n>0) generate(res, "", n, 0, 0);        return res;    }};