text: #include <vector>
  
  class Board {
  public:
  	Board(int size);
  	int size();
  	void place(int row, int col);
  	int unplace(int row);
  	bool is_ok(int row, int col);
  	bool solve();
  	int col(int row);
  
  private:
  
  	int size_;
  	std::vector<int> pos_;
  	std::vector<bool> col_;
  	std::vector<bool> diag1_;
  	std::vector<bool> diag2_;
  };
  