22. 括号生成

class Solution {
  vector<string> Result_;
  string Current_;

  void DFS(int Left, int Right) {
    if (Left == 0 && Right == 0) {
      Result_.emplace_back(Current_);
      return;
    }

    if (Left != 0) {
      // Still can start new pair by using '('.
      Current_.push_back('(');
      DFS(Left - 1, Right);
      Current_.pop_back();
    }
    if (Left < Right) {
      // Must close the left paren by using ')'.
      Current_.push_back(')');
      DFS(Left, Right - 1);
      Current_.pop_back();
    }
  }

public:
  vector<string> generateParenthesis(int n) {
    DFS(n, n);
    return Result_;
  }
};