code for generating all valid parenthesis.

parenthesis.cpp

class Solution {
public:
    vector<string> ret;
    string s;
    void rec(int a,int b){
        if(a<0 || b<0 || a>b) return ;
        if(a==0 && b==0) {
            ret.push_back(s);
            return ;
        }
        s+='('; 
        rec(a-1,b);
        s.pop_back();
        s+=')';
        rec(a,b-1);
        s.pop_back();
    }
    vector<string> generateParenthesis(int n) {
        s=""; rec(n,n);
        return ret;
    }
};