use char type for single characters

Lines 20 to 22 in 9e8eac2

    
           const string ROW_CHAR = "r"; 
        
           const string COLUMN_CHAR = "c"; 
        
           const string Diagonal_CHAR = "d";

Also "Diagonal_CHAR" should ne in ALL_UPPERCASE 😏

	const string ROW_CHAR = "r";
	const string COLUMN_CHAR = "c";
	const string Diagonal_CHAR = "d";