Generate Parentheses

Given an integer N representing the number of pairs of parentheses, the task is to generate all combinations of well-formed(balanced) parentheses.

• Time : O(2^n)
• Space : O(1)

class Solution {
    vector<string> res;
    void helper(string& build, int open, int close, int n) {
        if(build.length() == n) res.push_back(build);
        else {
            if(open) {
                build.push_back('(');
                helper(build, open - 1, close, n);
                build.pop_back();
            }
            if(close > open) {
                build.push_back(')');
                helper(build, open, close - 1, n);
                build.pop_back();
            }
        }
    }
public:
    vector<string> AllParenthesis(int n) {
        res.clear();
        helper(string() = "", n, n, 2 * n);
        return res;
    }
};