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:

[  "((()))",  "(()())",  "(())()",  "()(())",  "()()()"]
answer:
class Solution {public:    vector<string> generateParenthesis(int n) {        vector<string> result;        string str = "";        addPar(result,str,n,0);        return result;    }        void addPar(vector<string> & result, string str, int left, int right){        if(left == 0 && right == 0){            result.push_back(str);            return;        }        if(left > 0){            addPar(result, str + "(", left - 1, right + 1);        }        if(right > 0){            addPar(result, str + ")", left, right - 1);        }    }};