[LintCode] Generate Parentheses 生成括号

class Solution {
public:
    /**
     * @param n n pairs
     * @return All combinations of well-formed parentheses
     */
    vector<string> generateParenthesis(int n) {
        if (n == 0) return {};
        vector<string> res;
        helper(n, n, "", res);
        return res;
    }
    void helper(int left, int right, string out, vector<string>& res) {
        if (left < 0 || right < 0 || left > right) return;
        if (left == 0 && right == 0) {
            res.push_back(out);
            return;
        }
        helper(left - 1, right, out + "(", res);
        helper(left, right - 1, out + ")", res);
    }
};

class Solution {
public:
    /**
     * @param n n pairs
     * @return All combinations of well-formed parentheses
     */
    vector<string> generateParenthesis(int n) {
        set<string> res;
        if (n == 0) {
            res.insert("");
        } else {
            vector<string> pre = generateParenthesis(n - 1);
            for (auto a : pre) {
                for (int i = 0; i < a.size(); ++i) {
                    if (a[i] == '(') {
                        a.insert(a.begin() + i + 1, '(');
                        a.insert(a.begin() + i + 2, ')');
                        res.insert(a);
                        a.erase(a.begin() + i + 1, a.begin() + i + 3);
                    }
                }
                res.insert("()" + a);
            }
        }
        return vector<string>(res.begin(), res.end());
    }
};