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) {
        vector<string> re;
        par(re,"",n,0 , 0);
        return re;
    }
    void par(vector<string> &re,string now , int n ,int left ,int right)
    {
        if(left == n && right == n)
        {
            re.push_back(now);
            return;
        }
        if(left > right)
        {
            if(left < n)
            {
                par(re,now + '(' , n ,left+1 ,right);
                par(re,now + ')' , n ,left ,right+1);
            }
            else if(left == n)
            {
                for(int i = right ;i < n ; i++)
                {
                    now+=')';
                }
                re.push_back(now);
                    return;
            }
        }
        else if(left == right)
        {
            par(re,now + '(' , n ,left+1 ,right);
        }
    }
};