Generate Parentheses

遍历：注意左括号要在右括号前！

class Solution {
   vector<string> res;
public:
    void set(int left,int right,vector<string>& result,string str)
    {
        if(!left &&!right)
        result.push_back(str);
        if(left>0)
        set(left-1,right,result,str+'(');
        if(left<right &&right>0)
        set(left,right-1,result,str+')');
    }
    vector<string> generateParenthesis(int n) {
        string s;
        set(n,n,res,s);
        return res;
    }
};