给出 n 代表生成括号的对数，请你写出一个函数，使其能够生成所有可能的并且有效的括号组合。
例如
给出 n = 3，生成结果为：
[
"((()))",
"(()())",
"(())()",
"()(())",
"()()()"
]
代码
class Solution {
public:
vector<string> generateParenthesis(int n) {
vector<string> res;
generateParenthesisDFS(n, n, "", res);
return res;
}
void generateParenthesisDFS(int left, int right, string out, vector<string> &res) {
if (left > right) return;
if (left == 0 && right == 0) res.push_back(out);
else {
if (left > 0) generateParenthesisDFS(left - 1, right, out + '(', res);
if (right > 0) generateParenthesisDFS(left, right - 1, out + ')', res);
}
}
};
