Generate Parentheses

https://leetcode.com/problems/generate-parentheses/

递归，用两个指标left和right来标记出合法规则，之后再left和right都达到n后，满足了可以输出的条件

此时，就可以把这个string压入到res中了。

class Solution {
public:
    void getElement(int left,int right,vector<string>& res,string str,int n)
    {
        if(left==n&&right==n)
        {
            res.push_back(str);
            return ;
        }
        else if(left==right)
        {
            getElement(left+1,right,res,str+"(",n);
        }
        else{
            if(left+1<=n)
                getElement(left+1,right,res,str+"(",n);
            if(right+1<=n)
                getElement(left,right+1,res,str+")",n);
        }
    }
    vector<string> generateParenthesis(int n) {
        vector<string> res;
        string str="";
        getElement(0,0,res,str,n);
        return res;
        
    }
};