22 Generate Parentheses

Given n pairs of parentheses, write a function to generate all combinations of well-formed parentheses.

For example, given n = 3, a solution set is:

"((()))", "(()())", "(())()", "()(())", "()()()"

class Solution {
public:
    vector<string> generateParenthesis(int n) {
        string comb;
        vector<string> res;
        if (n<=0)
            return res;
        help(n,comb,res,0,0);
        return res;
    }
    void help(int n, string & comb, vector<string> &res, int leftCount, int rightCount)
    {
        if (leftCount==n && rightCount ==n)
        {
            res.push_back(comb);
            return;
        }
        if (leftCount<n)
        {
            comb.push_back('(');
            help(n,comb,res,leftCount+1,rightCount);
            comb.pop_back();
        }
        if (rightCount<leftCount)
        {
            comb.push_back(')');
            help(n,comb,res,leftCount,rightCount+1);
            comb.pop_back();
        }
    }
};