Nothing fancy, just use recursive.

1 class Solution { 2 public: 3     void getP(vector
     
       &result, string current, int left, int right) { 4         if (left == 0 && right == 0) { 5             result.push_back(current); 6             return; 7         } 8         if (left > 0) { 9             getP(result, current + '(', left - 1, right + 1);10         }11         if (right > 0) {12             getP(result, current + ')', left, right - 1);13         }14     }15     vector
      
        generateParenthesis(int n) {16         vector
       
         result;17         getP(result, "", n, 0);18         return result;19     }20 };